Yeah, I think Monster's Inc. is great and I was all for the idea of a sequel, but I was NOT looking forward to the prequel idea.
But I sort of love that they decided to go with a "Snobs vs Slobs" / "Animal House" / "Revenge of the Nerds" type-plot. To me, the key to a successful prequel is that since you are stuck with a story that you already know the ending of (in this case: Mike and Sully as friends and partners @ Monster's Inc.) what you need to do is get the story to that point in a way that is NOT what you would expect.
Everything I see about this movie seems unique to the first movie, expands the monster world with lots of new characters, and will hopefully be entertaining. As you point out, without Boo, they're still the heart and emotion piece that might be missing - it will be hard for them to tug our heartstrings with the fact Sully isn't going to be a scarer when we KNOW that he in fact is - so I don't expect it to be as good as the original, but it still could be a good time.
As you say, there doesn't seem to be a lot of decent family fare this year - though our family will also be looking forward to Despicable Me 2 - though so far the ads for that give me little hope. The original movie was funny partly because of the minions, but was great because of the "family" that formed with Gru and the orphans. The trailers so-far are all-minion, and while funny and appeal to my daughter, don't really appeal to me.
SP