So I was looking at the cast for the new Star Trek movie on IMDB.com and while I'm sure other people could dissect this more efficiently than I can, I'm gonna give it a try regardless.
Kirk: Chris Pine. Pine is essentially a no name actor having appeared in two moderately successful films in supporting roles: Princess Diaries 2 and Smoking Aces. Nothing in his career has shown that he can play Kirk, of course nothing in his career has shown he can't play him either and Shatner was a no name when he took the role on too. I expect he'll play it more straight up, but without a more thorough filmography, it's impossible to say.
Spock: Zachary Quinto. Quinto is most known for his role as the supervillian Sylar on the hit TV show Heroes. Of the chosen actors he certainly is the one that most looks like his predecessor, he's also the one most familiar with fandom. I'm sure he appreciates the somberness neccesary to play the role effectively and unlike a lot of people the history that goes along with it.
Uhura: Zoe Saldana. Well, she's certainly hot. I've no idea what Abrams intends to do with Uhura I can't imagine that all he'll have her do is sit around and answer the space phone in the 21st century though. She played a minor part in the first Pirates of the Caribbean movie, and that's the total of her filmography that I'm familiar with. That said her role in that movie was physical and comic, she should be fine.
McCoy: Karl Urban. Urban is the one I feel most comfortable with in his role even more so than Quinto or Cho. He's a good actor with an ability to subsume himself in his character. His background is filled with solid supporting parts, he can't seem to carry his own movie as evidenced by Pathfinder, but he does very well playing the second lead. I imagine that his McCoy will be a more physical presence than the Kelly one ever was since Deforest Kelly was roughly 145 years old when he got the part of McCoy.
Sulu: John Cho. Cho is most well known for his title role in the Harold and Kumar stoner comedies. Abrams and Cho seem to be sincere in their desire to portray the characters in the intent that they were originally written, as opposed to the more campy version that made it to tv screens in the 60's. Of course, we also live in a more congealed society than we did even 50 years ago.
Scotty: Simon Pegg. I don't get this. I like Pegg, Hot Fuzz might be one of the funniest movies made in the last decade at least, but I don't get him as the wily engineer. This may be more a failing on my part than the part of the actor though.
last we come to...
Checkov: Anton Yelchin. At least they got a Russian guy to play the Russian Ensign. Yelchin is in two movies that are on my to be watched list, Alpha Dog and Charlie Bartlett, however I've never heard of him before now. His part is probably the least important of the main characters, and as such it's not like he was Chris Pine.
Anyway that's the thoughts I have on the major cast member choices made in the new Star Trek movie outside of Eric Bana and Winona Ryder whom I believe are both decent actors, but whom comparisons can't be made about since their characters are all new.