From what I've seen:
Mallett is my favorite at the moment. Early on in the season, I really wasn't a big fan, especially after the Alabama game. He seemed to lose his composure at the end and cost his team with key interceptions. He really improved as the year went on and I thought he looked really good at the end of the year. If he's going to succeed in the NFL, he's going to need a good offensive line, period. He's not mobile at all and I don't see him as a Big Ben where he's big and and move around a bit with his size. His completion percentage is great this year even though it's also a product of Petrino's system, he still deserves credit for the improvement. He obviously has a complete cannon and simply put can make some amazing throws. He's going to need a fair bit of work to improve his mechanics and footwork but there's nothing that seems like it's beyond repair.
Cam Newton is also someone that's grown on me. When you first see him, you assume he's simply another running quarterback who can occasionally throw but Newton has impressed me with his arm as well. He actually has a really nice arm. The system he plays in is obviously pretty worrying, he'd have to adjust to a pro style offense. That being said he just oozes of talent and charisma (I think he'll do great in pre-draft interviews, etc.) and I really think it would be an upset for him to fall out of the top 10 picks at this point. Whether you think he deserves it or not, there are teams that are going to fall in love with this kid and might even have him over Luck. I'm not his biggest fan but I wouldn't be hugely upset or anything if he ended up here.
Jake Locker is someone I haven't watched a lot. To put it frankly, his numbers have been completely awful at times this year. In particular, his stats are terrible against the three best defenses he played (Nebraska x 2 and Stanford). There are a ton of excuses for Locker's bad numbers that are completely legit excuses but it's still a cause for concern. This isn't a developmental guy in the mid-rounds, we're talking about a potential top 10 pick here, I don't think its unreasonable to ask for some numbers. When you watch him, you come away impressed. He reminds me of Steve McNair actually. Still, I really need to see more of him...the Senior Bowl is going to be huge for him since he'll have talent around him.
Blaine Gabbert is again, someone I haven't watched a lot. From what I've read he seems like Newton and Locker...he has all the physical tools you could ask for. He's big, quick, and has a strong arm. Also like all the others, he's going to be a project that needs coaching in a pro system to be successful. Ok, yeah I don't really know much about him because I didn't think he would leave. It seems like scouts and the NFL are a lot higher on him then the Internet sites realized. I'm surprised he didn't stay as I think him and Matt Barkley would duke it out for #1 overall next year. Can't blame him for striking while the iron is hot.
So yeah, basically all four have awesome physical tools but come from poor pro systems or have bad mechanics. Having someone who can develop them is going to be key for all of these guys.