I routinely enjoy novel with disreputable, shady, or even downright villainous protagonists, but they need to either be sympathetic in some way or I fully expect the author to understand that they are unsympathetic (and therefore not expect me to like them). Lev Grossman doesn't seem to get that. He wants the reader to identify with Quentin, but Quentin is an irredeemable piece of shit nobody should want to identify with.
His actions only seem to have negative consequences for other characters. He acts like a dick toward pretty much everyone who befriends him. He indirectly kills a student by letting Martin Chatwin into Brakebills. He is arbitrarily cruel to Alice (who would have made a much better main character, by the way) and eventually gets her killed too.
...and he gets away with it. Maybe he feels bad about what he's done, but it doesn't matter. I don't sympathize with him, so I don't care about his feelings. Perhaps Quentin becomes more tolerable in the sequels, but I don't think I'd take the risk to find out.
The frustrating thing is that a Harry Potter/Narnia mash-up, populated with real people, rather than fantasy archetypes, is a pretty cool idea. In the hands of a master satirist, such as Terry Pratchett, it could have been both hilarious and insightful. Unfortunately, it turns out to be neither.