I've always loved D&D. I started in 1983 with 1st Ed AD&D (orange binding) and have played every version so far. D&D has a lot of things that I enjoy as far as flavor goes. Vancian magic, named spells, certain monsters and the way some things have always worked.
Now I also prefer the rules to be light. 1st Ed AD&D was pretty simple. The THAC0 thing was a little cumbersome, but otherwise it was pretty straightforward. Still had all the bits of D&D I love.
2nd ED was largely the same, though it did start adding a lot of optional rules which IMO kind of muddied the waters as far as simplicity goes, but the core of it was largely the same as 1st Ed.
3rd Ed (and 3.5) introduced some serious alterations to the game. Most of the original flavor was there, but there was a definite shift towards a balance between Player and GM. Lots of new toys for the players to tweak and adjust their characters. Some of those changes though I have always chafed at.
Largely...the Attack of Opportunity. Tied to the AoO is the grid and movement in squares...which has always brought me to an uncomfortable cross between minis game and RPG.
I love minis. I have several minis games. That said, I use minis in RPGs only in a very limited sense. I don't use squares, measurements are approximated and physical representations with minis are just for a vague overall impression of the scene.
Not using a grid eliminates the AoO and a dozen feats.
The reasoning for this is that use of the grid, 5-foot steps, measurements, really takes away from what I'm trying to achieve in a game that I'm running. I don't want a minis game...I want a fast and lean fight (when it comes up) with as little system interaction as possible. I want the focus to be on "I slide under the ogre's legs and stab him in the jimmy!" not I make a tumble check, I have mobility so I'm not incurring an AoO, and I have a bonus to my attack because of X, Y and Z.
Do AoO happen? Sure. I keep it as a tool for GM use when PCs do something cool or stupid, roll well or botch really badly.
"Ooh, the orc charges over the table (clatter of dice) and ooof, he trips, landing at your feet, sword flying away...you get a free shot at him!".
Out of the original 3.5 List of Feats, I've edited a total of 14. most are regarding AoO. A couple are regarding Dodge, which I've always thought was kind of ridiculous (Dodge is something anyone should be able to do IMO).
Another area of editing was in the area of Actions in combat.
Largely it's the same, but I handle it a bit more simply and "hand-wavy". In a nutshell you can move and act in your action. Some things like loading a crossbow or casting spells take all your effort to do, so you forgo one of the two. That's about it. I hate getting bogged down in "you can take a move-equivalent action, a normal action, and oh, you have that quickened so that's a Move equivalent too...bah!" screw that. [wave hands] Good enough, move on.
Other than that D&D 3.5 is intact. Metamagic feats are a bit cumbersome, but if someone ants to use them, so be it.
I know, these edits a lot of folks won't dig. That's cool. Welcome to my homebrew!