I'm not anti-d20; I think it's a well-designed system, and I'll continue to use it for certain types of games: mostly one-shots or mini-campaigns. However, I'm moving to a rules-lite approach for my main game. Here's why:
1. Time for play
IMC, combat took too long. We could be on one combat for the entire session, or we had marathon sessions that lasted well into the night. My group is all married couples with kids, and we couldn't do that, anymore.
2. Time for preparation
As a DM, preparing for the game took too long. Creating NPCs, applying skills & feats, templates, picking classes, et cetera eats up too much of my time. Even using modules is time consuming, because it's sometimes harder to get your mind wrapped around the adventure and the capabilities of the NPCs and monsters when you didn't create it/them.
3. Coarse-grained vs. Fine-grained
For PCs, I really like a coarse-grained approach to skills & abilities. I don't see this as an imagination vs. rules issue so much as a question of flexibility. As a DM, I prefer to make a judgment call on whether a character can do something or not, rather than having this dictated by the rules. Now, clearly, this isn't a black-and-white issue, but a balancing act. You have to have some rules and constraints, obviously. The question is where on the continuum you want to be.
I like coarse grained because your PC's capabilities are defined very broadly. A good example is the skill-bundle approach used in Lejendary Adventures. For example, say your character concept is a noble chevalier/knight. You might select the skill-bundles for Chivalry, Weapons, Physique, Hunt, and Learning. Chivalry includes things like riding, courtly behavior, heraldry, knowledge of fortifications, leadership, diplomacy, et cetera. Weapons includes skill with both weapons and armor and the repair of the same. And so on. In d20, you'd need a huge list of skills to cover these capabilities, and you'd probably end up with a character that lacked some of the skills that a person of his background/concept really should have. The obvious d20 answer is the use of such skills as untrained (i.e. ability checks), but the rules constrain this, in many cases. For example, knowledge skills are not usable untrained, by-the-book, so you're unlikely to have everything. In a coarse-grained system this isn't anywhere near as likely to be an issue; the DM makes the call on whether your character has the capability, based on broadly defined attributes (like skill bundles), or on an archetype, or just based on your character's background, depending on what system you're using.
A similar situation exists with d20 feats. Feats are pretty cool, but because they are all defined, it means you have to possess the feat in order to perform that action. For example, what if a player says "I want to swing really hard, maybe giving up some accuracy for power." Well, that's a power attack. In a coarse-grained system, the DM can say "okay, take a penalty to hit and we'll give you a bonus to damage if you succeed." In d20, the DM says "Do you have power attack?" If you don't have it, then by-the-book you can't do it. Same goes for things like Combat Expertise, et cetera. As a DM, I prefer to exercise judgment about such things, rather than being the rule-looker-upper.
Now, the obvious response to this criticism is "rule zero -- you don't have to use it." Okay. But when you do that, you unbalance a carefully balanced edifice of rules, and you also end up punishing PCs who may have been built using the rules (what use is the feat if anybody can do it), and you piss off players who feel cheated because you're not following the book. In that case, why not use a system that encourages the kind of game you want?
My point is that rules are cool, but the more of them that you have, the more constrained you are. For me, the preferred balance point is fewer rules than d20 provides. Coarse-grained means more flexibility, and it means faster play and preparation. All those are important to me for my main game.
I also think that coarse-grained systems help a DM become a better DM. For one, the rules are easier to master, which means a DM has more time and energy to work on story, setting, et cetera. Also, it encourages the DM to use his judgment and common sense, rather than searching for the applicable rule. IMO, that makes for a better DM, and that makes for a better game.
I'm not trying to say that d20 sucks. I don't think that. As I said, I think it's a well-designed system, and I'll continue to use it in my gaming; I have a ton of d20 material, and I do think d20 fits certain types of scenarios and campaigns well. But I think that the lite systems have something to offer, too, and it fits better with the way I like my main game to run.
If you haven't tried a rules-lite system, you might give one a shot. I know it seems like d20 can do everything those systems do, and in theory, I agree. In practice, however, I find that the rules-lite systems have a different style and feel; it feels like a different kind of game. Try it, you might like it. You don't have to pick one approach or one system; there's room for more than one around the gaming table, IMO.
p), much customisation - like feats only less workable 
