As I've mentioned before, these days I'm running my game using the Swords & Wizardy rules, modified fairly extensively by an increasingly huge bunch of house-rules.
I've played with a fair number of RPG rulesets over the years, and have never yet found one that is perfect. A perfect ruleset, in my mind, would be one that can be learned well enough in half an hour for fun gaming purposes by a complete neophyte (while creating their first character), while sumultaneously allowing absolute flexibility and covering every conceivable eventuality in every conceivable genre. Surely that's not too much to ask?
My first gaming experience was with AD&D in 1981, and that has coloured my gaming preferences ever since. I loved the D&D milieu, but became increasing frustrated with the restrictions imposed by the game framework — restrictions like making it effectively impossible for any character to pick a lock or a pocket unless they actually had a level as a Thief, or to track anything unless they were a Ranger, and so on. They were restrictions that were implied rather than stated; nowhere (that I can recall) did it actually state in the Players Handbook or Dungeon Masters Guide that nobody else could attempt these tasks, but the fact that only Thieves had a specific success or failure mechanic stated implied that, since nobody else did, only they had any chance of success at thiefly activities.
All this is a consequence of the design decisions Gary Gygax made when he was putting together the altogether more regularized AD&D after the earlier, more free-wheeling years of OD&D. I have no idea whether he intended those systems to be proscriptive, but that was the effect.
Anyway, around the time that I started playing AD&D, Champions — the point-buy skills-and-powers based superhero RPG that eventually turned into the Hero System — was first published, and I was introduced to it by my friend Mark. It was a little while before I decided to adopt it myself for my fantasy campaigns, but adopt it I eventually did. With a break of a couple of years when D&D3e came out, I used Champions, and then the Hero System pretty much exclusively for all my campaigns — fantasy, space opera, cthulhoid horror, western, and so forth. I used it for longer than any other single system; in fact for longer than all other systems combined. I must admit though, that when it came to fantasy roleplaying I spent a lot of time trying to replicate the D&D experience.
The construction-kit nature of the Hero System meant that if a player wanted to build a lock-picking tracker with spell-casting abilities, they could. That seemed to me to be a huge advantage. There was hardly a situation I couldn't deal with using the rules as written, with the exception of absolute invulnerabilities (the Hero System doesn't do absolutes), and they were easy enough to work around. However, some situations required some pretty creative accounting with the construction rules to accommodate.
So why change?
Mainly because of that word — accounting. Character and monster building ended up becoming an exercise in number-juggling; sure, there was an initial creative spark required to come up with the concept, but then it was just a lot of fiddling and optimizing and accounting to build the thing. Later on, when it came time for the character or monster to meet each other in mortal combat, there was a whole lot more accounting to be done to determine the outcome. It's customary for proponents of some other RPG systems to mock Hero for the supposedly excrutiating length of time it takes to resolve even a few game-minutes of combat; that mockery tends to vastly overstate the actual complexities involved (and there's a pot-kettle situation there too with D&D3e — attacks of oppportunity or grappling, anybody?) but the criticisms do contain a kernel of truth.
In an attempt to explain and regularize the rules more fully, in theory (I think) to make them more accessible, the Hero System has become HUGE. The 5th edition core rules were a massive tome heavy enough to form their own gravity well; the new 6th edition is split into two volumes, each of which is about the same size as the whole 5th edition book. Then there are genre books and other supplements, some of which are almost as extensive as the core rules. That's a lot of information to absorb, even for someone as familiar as I am with the rules, and for a newcomer to roleplaying it's flat-out terrifying.
D&D has suffered from the same syndrome. In an attempt to regularize the rules so that there can be no ambiguity (a goal which has not, in my opinion, come close to being achieved even yet), and to cater to wider and wider player desires and expectations, the rules have become bloated beyond belief. The rulebooks certainly look a lot prettier and more colourful than they used to back in the '70s, but they've also become a lot less accessible.
A common criticism of players of older editions and their clones — or observation, call it what you will — by proponents of the more recent versions of D&D is that we're only playing them out of some misguided sense of nostalgia. Taking aside the pejorative intent, in my case that's true, but only to an extent. I'm not trying to recapture the glory days of my youth in roleplaying; that's never going to happen. The joy and wonder you experience when first learning to handle this rather bizarre hobby isn't replicable unless you suffer some kind of brain injury and forget everything you've already experienced. I am trying to replicate the experience of playing D&D, just as I did when I was using the Hero System exclusively, and that experience isn't satisfied (for me) by D&D3e, and even less by 4e.
The reason I'm playing Swords & Wizardry instead of AD&D is because, as I mentioned before, I found AD&D too proscriptive. I could be using the original little OD&D booklets, but frankly I find their organisation and layout awful. I was tempted by the D&D Rules Cyclopaedia (of 1991, I think), but by the time I got a copy I'd already pretty much sorted out my house-rules for S&W (and I do prefer ascending AC) so there didn't seem much point in changing the campaign over again. S&W gives me the D&D experience I'm looking for, but with more freedom of action for both players and DM.
Is it the perfect fantasy roleplaying ruleset? Not yet. That's why I keep tinkering with it. Maybe one day...