The design work that went into 4e created a whole new game. There's a lot to like in the new game, and a lot to hate. This conversion document is going to ask the question:
What if all that design work had gone into fixing 3e instead of making a brand new game?
I'll organize it by Book and Chapter over on the sidebar, so if you click on Feats you'll see how feats should change if you want to use 4e stuff in your 3.x game. If you choose spells, you'll probably see a bunch of things as they get added. I'm encouraging people to add whatever conversions they like!
Here's an example:
Cleave: Cleave pretty much sucks in 3e, you spend a feat in order to gain an extra attack maybe a dozen times over your adventuring career. Big whoop-dee-doo! 4e's version of Cleave is much better, and there's no reason the 3e version couldn't be as well.
Prerequisites: Str 13, Power Attack
Benefits: Whenever you make a successful strike, deal your Str modifier in damage to an opponent adjacent to your original target.