Well, with the lack of opportunity attacks, a lot changes. Positioning is relatively unimportant, so you can even play without a map if you want, although many people will still want to do this so they can visualize what's going on.
Of course, a lot of standard d20 strategy is meaningless without AoO, feats such as Mobility and Combat Reflexes either need to be changed or removed completely, and you never need to make a Concentration check or take a 5' step in order to successfully cast a spell. In fact, the old 5' step becomes pretty much frivolous in True20.
There are of course, some of the standard considerations, such as flanking, that still exist in the system. And there are feats and abilities, such as point blank shot and sneak attack (ranged) that depend on range.
I'm still learning True20 myself, so if anyone else wants to chime in, that would be great, Plus, I'm at work and don't have my book in front of me.