You really have to try Bouchon in the Venetian, from what I've heard. That's the Thomas Keller restaurant.
The 2 best buffets for breakfast are apparently at Paris and Bellagio. We loved the one at Paris, but didn't try Bellagio. We went off the strip for our other breakfasts. One place was just a cute, good, breakfast joint called The Egg and I. The other was Hash-House-a-go-go, which was also good but the real draw for this one is the humongous portions! One of our friends ordered granola because she didn't want a huge breakfast with eggs and pancakes and such, but the bowl was still extra-large. It's a lot of fun :)
We had a nice dinner at Trevi in the Forum Shops area - the food was pretty good and reasonably priced and the atmosphere was great - simulating an outside patio in Italy right by the fountain (the fountain is quite loud, though).
Our other dinner was at the Wolfgang Puck restaurant in the Venetian, which also has a wonderful atmosphere in the patio area, but the food didn't live up to celebrity status. There is a pricier restaurant next to it that might be better, but if you don't have high expectations this one isn't horrible. It would be good for lunch if you didn't pig out at breakfast :P We had a really great South African white wine here (buitenverwachting, constantia 2005).
That's all that we tried, but I know there are plenty of others that are worth trying! Joël Robuchon, Alize, ones in the Rio and hotels around it - one of them has a restaurant up high with a view of the strip and it sounds like their restaurant/nightclub are quite trendy and popular. There are some restaurants away from the strip that are very popular with locals... an authentic mexican one, Pamplemouse (french), and a couple of others.