I don't know anything about the Russian players, but I've seen each of the Americans play.
Querrey I can see actually entering the top 10 at some point if he polishes up his game and starts to really believe he belongs there. He has such an imposing game that he can compete with anyone if he's playing well. However, I think he's still getting used to being a pro, although he is beginning to show that he is no longer content with just playing at the ATP level. I think we're going to see him be hungrier, more motivated, and less mellow in 2009. He's been working on his movement, which is great, but I'd really like to see him improve his volleys. Being able to successfully follow one of those strong serves or forehands to the net would be a huge advantage, preventing his opponents from being able to work their way back into a point and put Querrey on the defensive, which won't consistently end well for him. My guess is that we'll see him move into the top 30 in 2009 and that he'll float around the 10-30 range for most of his career.
Levine needs to bulk up, plain and simple. He'll never compete for slams, but there's no reason that he can't get inside the top 50. His play style is a mix between Nieminen and a physically weaker version of Hewitt, and he'll be helped by the fact that he can play well on both fast and slow surfaces. However, as with most counter-punchers, with some muscle he'll be able to dictate points better. I wouldn't be surprised if he has a career like Nieminen, performing consistently and floating around between 15 and 50 in the rankings. However, he's never going to have a huge weapon to rely on.
I don't know what to think about Isner. He showed some real promise last year but really sucked in 2008. His success relies almost completely on his ability to serve well, and like Karlovic it may take a while to become mature enough to understand when/where he must focus and to handle the pressure of having to do one thing very well. He dealt with that very well when he first went pro, but I think in 2008 the pressure and expectations got to him. As a result, his ability to rely on his serve declined, hence the worse results. Also, his ground game is terrible (in case you didn't know
). He's going to have to serve and volley on almost every serve, which I'm not sure he did this year (he did it a ton in the summer of 2007). I watched him play Andreas Beck in the USO 1st round and he did it maybe a quarter of the time. As a result, he was facing a ton of pressure during his service games. He can't play like that and expect to do well. He'll probably consistently remain in the top 100, but I can't see him making it to the second week of slams very often.
Young has all sorts of talent, but like Levine he needs to put on some muscle. Plus he needs to mature. That's pretty much it with him. He's way to streaky right now to do anything spectacular in a slam, but if he matures and bulks up I could eventually see him competing for slams. He gets a lot of criticism around MTF (and in general, but especially on MTF) for being a headcase and being a waste of talent. However, once he matures a little I think this will change. He's still very young, so it'd be stupid to write him off as a top player this early (the only player younger than him who is ranked higher is Nishikori).