I started playing AD&D (2nd Edition) as a kid. It's not a great system, but it was all I knew at the time. I ran games for my friends and we didn't pay much attention to the rules. When 3.0 came out, we were excited because Feats looked like a good way to design characters who were, well, heroic -- they could do awesome things according to the rules. Needless to say the early promise of 3e was never fulfilled for me, but I was playing other systems and so I just put D&D on the shelf.
Later, in college, I hauled out my 3e books to try to run a few games. In the intervening years I learned a lot about how rules work and, indeed, that good rules can make a game better and bad rules can make it worse. Rules are the backbone of any RPG and while you can just "house rule" around them, it was usually better to just use a system that did what you wanted it to do. At the time this knowledge was still in its infancy, but picking up 3e after all those years did a lot to rekindle it. I saw a lot of pointless rules (e.g. Profession Skills) and a lot of badly written rules (e.g. Grappling) all of which I tried to deal with on the fly. As a DM this was a burden because I needed to make the game make sense for my Players and the books I had paid for weren't helping. The real shock was when I had to deal with a munchkin who ran a 3.0 mid-level Wizard loaded up with scrolls and such -- he trivialized the adventure until another Wizard fried him to a pulp. I swore off running 3e at that point -- the rules didn't work for my Players, and they sure as hell didn't work for me.
A couple of years after that I was invited into a 3.5 game. I was in the middle of a RPG famine and, while I thought 3.0 was silly (and had the same opinion of 3.5 via the SRD) I figured I'd give it a shot. I rolled in with Twitch, a Human Fighter/Rogue and spent much of the early building time chuckling about the sillier points of 3.5: taking Rogue 1 for the skill points and later Fighter 1 for weapons and such seemed like laughable metagaming to my mind raised on the "Class is Life" that was AD&D; managing skill points and modifiers based on a Take 20 check or for synergies rather than what my character should know. To many veteran 3.x Players I'm sure this seems trivial but as I had been leaning towards the more rules-light games up to then it caused me to approach the whole "character building" in 3.x as an absurd mini-game. Still, it was a great campaign thanks to the RP and storytelling of the DM but towards the end (i.e. mid-level play) my humor turned darker.
This was the game with the novice Player (she had only ever played Diablo before) who decided to play a rather greedy Cleric. It was good fun but she had no idea how to deal with a prepared caster. Well, she liked having pets so I took a crack at the Monster Manual and realized that Celestial Bison were ludicrously good for combat (it was just Twitch, a kobold Sorcerer, a halfling pistol rogue, and the Cleric) and that Celestial Monkies were ideal for trap springing. In fact, Rogues are terrible for trap-springing as most traps target flatfoot AC rather than Reflex and my foot-pad soon took to hauling around a suit of heavy armor in case he was on "trap finding" duty. Having failed as a traditional thief, I had hoped Twitch might still be a good brawler yet as we increasingly faced foes with magic (e.g. Flight and Hold Person) and the undead I began to realize that Twitch didn't stand a chance in these sorts of fights. He was neither Fighter nor Rogue and the Cleric was both.
In the very last sessions of that campaign Twitch spent most of the fights sidelined. In one he was gazed at by an Umber Hulk and simply could not make a Will Save (Fighters and Rogues being awful at such) while in the final one I managed to carve up two minions before getting Held (and saved from being Couped by the mercy of the DM) and later Confused. During this the n00b Cleric was flying around, smiting enemies and basically holding her own against the assembled foes. It made the campaign bittersweet, and yet these moments were not the fault of the DM -- they were the fault of the system. 3.x privileges magic over mundane to such a degree that the most mundane of classes cannot function on the same battlefield as a caster once you hit levels 5-8; indeed, the DM (an experienced fellow) said he would not run a game past LV 8 because he could not handle the power levels.
I had sworn off 3.x at the point (or thought I had -- I later played a heavily homebrewed 3.5 game that was a disaster due to the DM, not the system) and went back to playing other games. Mostly, this meant taking up Indie RPGs like Bliss Stage and Mountain Witch -- games where the rules are simple, clear, and functional. Bliss Stage, for example, purports to be a game of high drama roleplaying and has rules that deliver on that promise. Here I really understood the value of rules not just to create a game, but to create a specific game, with specific gameplay and themes. You saw this to a lesser extent in some of the older systems but usually the core themes and mechanics were obscured and polluted by a crust of "realism" or ancillary concerns that were bolted onto the system for no real reason. Shadowrun, for example, purports to be a game of cloak & dagger intrigue in which high-powered criminals fight a shadow war for powers that be; the mechanics (pre-SR4) are exclusively concerned with killing things and blowing them up. While this experience was revelatory to me, there was not yet an example of this principle of design being applied to any mass-market game.
Then came 4e. Now, I mostly skipped out on the fanfare and I probably would have ignored it were it not for the fact that I was invited to play in a game of it. This was with a new DM who was new to roleplaying in fact, but because it was game, and I rarely turn down a game, I accepted. He passed onto me the 4e Core books and I read them over and delighted. Gone were the clunky wordings of 3.x and instead there was a streamlined system that did what it said. It helped, of course, that Classes were Strong again (a fact I missed from AD&D) but more importantly the various metagame features of 3.x I so disliked were gone. Thieves were good at thieving; Fighters were good at fighting; and everyone could contribute to the battle. The more I read about the rules the more excited I became -- this was a system I could play without constantly clarifying and amending rules; it was a system I could use.
I played the heck out of 4e in the following years -- primarily as a Player but later I got back into my familiar DM's chair and really stretched my wings. I introduced new gamers to RPGs with it, and was able to refine my storytelling abilities while the system basically ran itself. I didn't need to spend hours stating up NPCs (or making a library of them), nor did I need to agonize over the make-up of every Encounter (the rules told me how to do that). When the rules didn't work quite as I liked them I was able to tweak them, yes, but that was the exception and not the rule.
When the Edition Warz broke, I first came in on the side of 4e, defending it against nonsensical complaint such as "it's just like WoW." Over time, I became curious as to why people would cling so fervently to what -- IMHO -- is a strictly inferior system by any measure of mechanical elegance you wish to use. As was once said, the designers of 4e had "done the math" of game design and produced a coherent system that facilitated a party of adventures going into dungeons and slaying dragons. A debater at heart, I engaged in a Socratic inquiry on these forums to better understand the 3.x proponents -- was there something in 3.x I had missed? After lengthy discussions I learned what held the fascination of many 3.x Fans: flavoring aside, they mostly enjoyed the intricacy of multiclassing and feat-chains, not to mention the raw power of certain classes. Neither mechanical complicatedness nor raw power held my fascination, so as the Warz died down, I moved on.
This does not mean I gave up liking 4e, nor believing that it is strictly superior to 3e from a systems perspective. Rather, I felt I had learned all I could about what drove people to continue with 3.x and knew that further questioning was unlikely to give me greater enlightenment. However, there persisted threads in which folks would gather to bash 4e. Mostly, I wouldn't bother to enter those threads unless the OP was asking as to whether they should play 4e or not -- as a fervent supporter of 4e, I think everyone should at least give it a try. There I would do my best to simply refute the false and highlight the good of 4e. Even here, I usually only swoop in when I feel a particularly egregious point is being made regarding 3.x or 4e.