The title says it all. TTW does a bit more than just simply combining fallout 3 and new vegas with the new vegas engine, it makes both games a bit more fun with the extra stuff like a few subtle references to new vegas in the fallout 3 portion (and vice versa but I'm not sure), rebalancing weapons, etc. I ask what things do you like that TTW does.
Personally, I like that you can choose to have new vegas style speech checks in fallout 3, it's so much better than percentages.
I also like the fact that enemies and vendors have fallout 3/new vegas weapons in both portions.
Lastly, bobbleheads, you can choose to have them boost your skills and special or boost stuff like health and damage, either of those are much better in the long run than getting caps from a snow globe. (Plus personally, bobbleheads are nicer to look at.)