Kemba Walker is undersized, but not in the way people think he is. He's actually not all that short for a point guard - 6'1" in shoes, which is maybe a tick below average but not greatly so. The problem is that he has a 6'3.5" wingspan, which is well below average. He isn't going to be using Rondo's poke-away-the-ball-from-behind trick very often, that's for sure. Other than that, he's much more athletic than people realized as well - his vertical is nearly 40 inches, which is well above what anybody expected coming in.
He reminds me a bit of Ty Lawson, as has been said several times. I don't think anybody expects him to be a first or second option on an offense - at least not an efficient one - but I do think he can be a good spark off the bench.
Also, don't underestimate Brandon Knight. He was a gunner his first year in college because that's what he was used to doing in high school - he had to do everything for his team at that level. As soon as he started learning to play real offense, he played much better. I'd expect him to continue to learn, because he's reputed to be an extremely intelligent person both on and off the court.
He's a project. He's only 19, he'll put on more muscle as he matures, and he doesn't have much experience running a real offense. He could have benefited with another year in college at most places, but with Marquis Teague coming in behind him I think leaving Kentucky was a pretty good decision. I think it'll take him a couple of years, but I fully expect Knight to become a solid starting guard - but I don't think he's an All-Star. I don't think anybody in this draft is, frankly.