Something i noticed that is a lot better in BFG is that the engine doesn't have that annoying stutter than Doom3 had every second. I think you could fix it with com_precisetic 0, but that often messed up the timing of the game.
When walking forward or whatever in BFG it is silky smooth.
I've also been really impressed with all of the lighting throughout the game. The Doom3 engine still is really awesome in this respect.
I really do wish they had put more effort into the multiplayer aspects though, and with Doom1 and 2. I'd love to play multiplayer doom1 and 2 through BFG, rather than trying to find a game through a third party server browser with a different source port.
Im really glad i got BFG and i hope that ID provide further support with dedicated servers etc.