D20's complication for me (that is why I don't want to run it anymore..or at least for the time being) is in it's tactical combat which bogs down as PC levels get higher (and more feats/special abilities kick-in), not to mentione it's near-reliance on miniatures and a grid (which I don't use) for anything but simple combats, as well as some other areas I would like to modify but which throw other systems out of whack( for example, the default "magic level"...when you tinker with this, you screw up the already very delicate CR/EL system, or unbalance the wizard/sorceror when you adjust item creation ).
But I definitely concur with the Col, about feats. I think many people just read the description, and think it's easy to get. Same with no ability score minimums..some of the old grognards (though I am one too) just don't see that , if for example, you don't have a high CHA, you are gonna be a pretty underpowered Paladin. They also don't realize that ALL ability scores now, are more crucial..for example.. A Cleric needs not only a High wisdom, but a High CHA as well.
Feats don't bother me so much when they are transparent during actual play, but many are not transparent during play, especially when it comes to monster feats and "special abilities...improved grab...constriction...how many things undead (for example) are immune to...The special abilites and conditions section of the DMG is a big headache for some.
3E/D20 can be very intimdating to run depending on what kind of players are in your group...especially if you like/need to tinker with it...
Playing the game is easy...