Agents and editors always say that when they read manuscripts they are looking for great characters to fall in love with -- on the whole, that seems to be the number one thing that determines whether they will or will not make an offer. And to a large degree I'd say they're right to feel this way. Certainly if a manuscript has weak, contemptible, or otherwise annoying characters then nobody is going to enjoy reading it; and if the characters are merely flat or stereotypical then it's going to take a real blockbuster of a plot to save the story. And when you come across a truly dynamic and delightful character... well, I can easily imagine how that would make a manuscript jump right out of the pile at you.
How often do you, in reading published books, actually fall in love with the protagonist? If you're an HP fan, for instance, did you keep reading HP/SS because you just adored Harry so much you couldn't put the book down until you knew what happened to him? Or is it really the whole idea of the story, of which the hero and heroine are just a part, that makes you keep reading?
Maybe I'm strange, but when I think back on all the books I've read and enjoyed over the past few years, I can only think of a handful of characters that grabbed me so hard I just had to keep reading to find out what happened to them. Even some of those took quite a while to grow on me -- it was the end of GoF before I "got" Snape, for instance -- and in most cases, the character I ultimately fell in love with was not the protagonist. More often than not, I fell in love with the whole atmosphere of the book, the concept, the execution, and all I really required of the protagonists was that they behave in ways that made sense and didn't make me want to bash their heads in with a mallet.
Admittedly, though, when I think of the books I've read recently that I disliked, in nearly every case it was because the characters annoyed me, rather than because of defects in the worldbuilding or the authorial style or whatever.
I dunno, what do you think?
P.S. My brother, who reads crime fiction, has just sagely observed that the importance of a really engaging/compelling protagonist to an enjoyable novel varies with genre. This is true. Let us assume, then, that the above remarks are limited to the fantasy genre in particular.