I been asked many times why I don't like later editions of D&D and still play older editions. I think this can best be answered by listing some of the major things I look for in a set of fantasy RPG rules.
1) I want rules-lite. I'm not interesting in hundreds of pages of rules. I don't want to waste time have to look up everything instead of just making a ruling on the spot. I don't want players to feel they have to study the rules to play well. I don't want them to even feel like they have to buy a copy of the rules to play.
2) I want simple, fast character generation. Character generation should take 5-10 minutes for a experienced player -- and not much longer for an inexperienced player with a more experienced player helping him. Players should not have to make lots of decisions at character generation. They especially should not have to make many decisions that, if they choose incorrectly, will hobble their character far into the future. I don't want a character building subgame -- especially one that gives a major advantage to players who buy, study, and master the rules.
3) I want simple, very fast playing, abstract combat rules. If minis and battle mats are needed or even strongly suggested, the combat system is probably too detailed and tactical for what I want. Players should not need to learn rules-oriented tactics for combat. Again, I want my players to be able to play without having to study and master the rules. Combat rules do not need to simulate reality exactly, but they should be easy to map to reality. Disassociated combat rules are probably the most annoying type of disassociated rules. Average combats should take 10-20 minutes maximum.
4) I want distinct "classes" that vary in ability and skills both in and out of combat. In other words I want magic-users, rangers, fighters, etc. to be actually different in play. I don't want every class equally capable in combat. Not everyone is interested in combat and combat is not the center of my games. Combat is also fast (see 3 above) so those who aren't good in combat (or who just do not find combat all that interesting) will not get bored in long, drawn out combats.
5) I wanted limited "skills" Many things like finding traps, negotiating with others, etc. should be actually role-played by the players. Saying "I check for traps," rolling a die, and announcing the result is boring. If skills are in the game they should not be usable to short-circuit actual role-playing.
6) I want easy to modify rules. I run my campaigns set in my own homebrew worlds. I change the game rules to match the needs of my worlds. I do not change the worlds to match the needs of the rules. This means the rules need to be easy to modify and not so tightly integrated that almost any change will ripple across the rules with unexpected side-effects.
7) I want generic rules. As I said, I'm interesting in running my fantasy RPGs in my own worlds. That means I want the rules set I use to be as generic as possible. I don't want the rules tied too closely to a specific world or even the designers' favorite style of play. The narrower the focus of the rules, the less likely they are to meet my needs.
8) I need verisimilitude. My game worlds need to feel "realistic" -- verisimilitude as opposed to actual realism is fine. Rules that clearly don't feel real (like only NPCs being able to buy magic items in AD&D or powers that work regardless of circumstances where logically they would not like tripping a gelatinous cube in D&D4e) are annoying. All rules sets have some rules that break verisimilitude (the feeling that the world is real), but the more such rules there are in a game, the less likely the game will meet my needs. Players should be able to easily describe what they are doing in terms of the world, not in terms of the rules. If they have to speak on think in "rules" then the game probably isn't going to work well for me.
What I look for in a set of fantasy RPG rules may be very different from what you look for in a set of fantasy RPG rules. There's certainly nothing wrong with that. However, I think what I look for in rules explains why I prefer older TSR editions of D&D to the WOTC editions. Just as what you look for explains why you prefer the games you do. There is no one true way nor one true rules set.
[Don't forget that Original Dungeons and Dragons Goodies Are Available (for Cancer Fund Donors). There is still plenty of time (over a week as I write this) to make a donation and get in on the giveaway. Thanks much to those who have already donated.]