First, a bit of history. Tunnels & Trolls was probably the second role-playing game that was published. Its author, Ken St. Andre, initially got his hands on the original D & D set (the three little brown books) and lacking the miniatures wargaming background assumed by those rules but still understanding and liking the underlying concept, came up with Tunnels & Trolls. The game was fairly similar to D & D, with three character classes (later expanded to four), with attributes rolled up on 3d6 and set in a similar high fantasy setting. The game is noteable for being one of the first to have a universal task resolution system which drove everything in the game save for combat. The combat system involved rolling dice and adding modifiers and comparing the "hit point totals" for both sides involved with the side with the lower "hit point total" taking the difference in hits, evenly distributed amongst the losers. Armor absorbed damage, and spell-casting was based on a point system. The version of the rules that most of us familiar with the game have probably been exposed to was the 5th edition which was published in 1979. What is notable about the game is that the very core of the game really hasn't changed all that much since the 1st edition was published in 1974.
The editions I am discussing are the 1st and 4th editions, which are available as reprints from Outlaw Press. By modern standards, these barely qualify as RPG's as they don't contain any background information, any play examples, nor an example adventure. However, it should be realized that save for some minor additions, these are straight reprints of the original printings of these games. Lack of background information, etc. was pretty much par for the course back in the early days of role-playing games. One need only look at the original D & D rules or the original three Traveller little black books to see this. The artwork in the early editions (the majority in the 1st edition is by Rob Carver while the 4th edition contains art by both Rob Carver and Liz Danforth) is pretty crude but serviceable. The familiar "You missed my vital spots" cartoon is there; it seems to date all the way back to the 1st edition.
What I found interesting about these early additions is that the core mechanism stayed the same going from the 1st to 5th editions yet there were some noteable differences. I shall summarize the differences below:
1) In the 1st and 4th editions, melee combat is handled as it is in the 5th edition except missile combat is handled very differently. In the 1st edition, missile combat is saving roll based with player characters having to make a saving roll on luck to avoid getting hit whilst the monsters have to make a saving roll to avoid the same fate. In other words, you didn't roll TO HIT, you rolled TO DODGE/AVOID. In the 4th edition, a little CRT (combat result table) is provided for missile combat in which you cross-reference your dexterity with range and find out whether you hit, missed or what number range you had to roll on 1d6 to hit. I found the 4th edition missile rules to be klunky but the 1st edition missile rules were pretty nice, especially since they gave 1st level characters a fairly good chance of hitting something (in the 5th edition rules, it was fairly difficult for a 1st level character to hit a man-sized target at point-blank range).
2) There were only three character types in the early edition rules: warrior, wizard and rogue. The rogue is a character with innate magical ability who can use weapons. What is interesting is that rogues could not advance past 7th level! Once they advanced past this point, they had to choose to either give up all magical abilities and become a 5th level warrior or give up all their ability to use weapons and become a 3rd level wizard! I believe these rules were put in place for play balance more than anything else; in the early edition rules, the warrior class doesn't really have much of a distinctive advantage over the others; in the 1st edition, his only "ability" is the ability to use all available weapons...however, the rogue can do the same thing...AND use magic.
3) All editions of the rules (1st, 4th and 5th) handled armor differently. In the 5th edition, armor absorbed damage and warriors got to mulitiply the damage absorbed by armor by two (this was their advantage over wizards and rogues). In the 1st edition, warriors got no such advantage; their only advantage was that they could use any weapon that they had the sufficient attributes to wield. The rules handled shields and armor differently; shields absorbed hits whereas armor that was worn took hits, i.e. suits of armor would absorb a certain number of hits and then become useless. I imagine this rule encouraged the use of shields and gave adventurers incentive to continuously purchase new suits of armor. The 4th edition handled armor differently yet again; like the 5th edition, all armor absorbed hits but warriors had the option of "burning up" their armor; they could have their armor (both shields and suits) absorb damage equal to their intrinsic protective value multiplied by their level after which the armor would be useless. I have to say that I like the rules for armor attrition contained in the early editions since they encourage characters to purchase shields (in the case of the 1st edition) and probably helps to avoid some of the stalemates between well-matched (and well armored) opponents that could happen in the 5th edition.
4) Weapons and armor in the 1st and 4th editions did less damage and absorbed less hits than in the 5th edition. To give you an idea of the "inflation" that occurred between the 4th and 5th editions, a typical sword in the early editions did around 2d6 damage (as opposed to 3d6) and a dagger did 1d6 (as opposed to 2d6). Leather armor absorbed 2 hits as opposed to 6 hits. And so on.
5) The spells are organized a little bit differently; the spell list in the early editions goes up to 17th level (1st edition) or 18th level (4th edition) whilst the spell list in the 5th edition goes up to 20th level.
6) The Monster Rating (the measure of a monster's meanness) is handled a little bit differently. Instead of the easy to remember system in the 5th edition whereas you divide by ten, round down and add one to determine the number of dice rolled by a monster in combat, you actually have to refer to a chart which doesn't seem to follow any discernable pattern until you get to a MR of 100 and above. The personal adds for monsters is handled differently as well; monsters get half their monster rating in adds only during the 1st round of combat after which they get 1/4th their monster rating. I imagine this would have a profound effect on combat since many monsters would come out strong in the 1st round and then lose much of their effectivenes in subsequent rounds. I guess those monsters didn't follow a exercise regime to beef up their cardiovascular endurance.
7) One of the most noticeable features in the early editions is that there is a system of saving rolls for monsters which are characterized by monster ratings. I have to wonder why this wasn't included in the 5th edition. In brief, there is a separate system of saving rolls which uses the monster rating as the characteristic which is tested. For example, to make a 1st level saving roll, a monster would have to roll 100 minus its monster rating on the same number of dice it would roll in combat. In the case of trying to avoid missile fire (in the 1st edition), the monster would have to roll over the sum of firer's dexterity and luck on the number of dice it would roll in combat.
8) The 4th edition contains optional rules which I'm really surprised didn't make it into the 5th edition since they address another one of the complaints about T & T's combat system; if one side is sufficiently outmatched, it has no chance of causing any damage to its tormentor. Frankly, I felt the was a non-issue. If another rules system were being used, the end result would still probably be the same except that with the outmatched side dying 100% of the time using the T & T rules set, the outmatched side would die "only" 99.99% of the time using another rule set, i.e. D & D, Runequest, etc. The bottom line is...if you're that outmatched, you should try something else! Anyway, where was I? Oh, yes - the optional rule. It was pretty simple...the loser in an exchange of blows in hand-to-hand combat would inflict 10% of the damage it took back at the winner. For example, if side A took 30 points of damage, it would inflict 3 points of damage on side B. Now, this rule was a little bit goofy (the more damage you took, the more you inflicted...interesting) but it did allow the losing party to get some licks in. Another alternate method which was suggested in the rules was to have the members of the winning side in an exchange make a 1st level saving roll on luck to avoid getting hit; if they did not make their rolls, they would take the difference in damage. The obvious problem with this was that in a fight between two fairly pathetic combatants, i.e. a 1st level warrior and an orc with a MR of 10, the fight could easily end with both sides hitting and kill each other.
There are other minor differences between the early editions and the 5th edition but these eight summarize the major ones.
How do I feel about these early editions? It's difficult to say since we're talking about rules that are more than thirty years old. However, there are some features they had which I liked compared to the 5th edition, most notably the missile combat system in the 1st edition, the system for monster saving rolls and how armor could be attrited. The optional combat rules in the 4th edition shows that the designer was at least aware of some of the purported problems with T & T's combat system although both of the suggested changes were probably rendered moot by the commonly used spite damage rule.
What is interesting to me is that the core system has remained virtually unchanged for over thirty years, which speaks to the robustness of T & T's rules lite approach. The foreward of the 1st edition is also noteable for mentioning Dungeons & Dragons specifically by name as the source of inspiration for T & T; apparently, legal niceties required Ken St. Andrew to refer to D & D obliquely in later editions.
Are the rules worth shelling out $16 a piece? Probably not. However, for someone interested in the history of the game, the rules certainly provide a lot of insight. Rules in the early editions which were dropped in the 5th edition also might serve to provide inspiration for house rules to aspirign GM's. The 1st and 4th editions are also where you really get a feel of Ken St. Andre's strange sense of humor and his approachable writing style; much of that was lost in the 5th edition due it actually having an editor; the 5th edition is clearer but I think some of the personality of the early editions was lost.

