I don't hate Hawke, in fact I like the character a bit, but the problem is that Bioware kept babbling on and dribbling over how Hawke was the most "important and awesome" character in the DA universe, which he/she clearly wasn't. Hawke was, for act 3 mainly, not plot integral at all because everyone still turns on each other and boss Orsino and Meredith attack Hawke despite which choice you made (Meredith fight when siding with templars can be excused through her insanity however but it still has no real justification or explanation behind the motive.)
In truth, the whole "rise to power" and "important character" aspects was just sloppy writing by Bioware. There was never any rising to power because even after I become a Champion, which is nothing more than a powerless title, I'm still doing the same quests, babysittings and chores I was doing in Act 1, there was nothing in DA2 to suggest Hawke had a rise to power, all Hawke became was an extra, useless voice in the argument between mages and templars. Speaking of which, if Hawke is supposedly the most important person in the world because of this then that is a terrible reason, because Anders was more important than Hawke because he was the one to set off the war. The only part where Hawke may seem *slightly* plot integral was saving Kirkwall from the Arishok, beyond this. Hawke had very little motives, reasons and justifications for what he/she was doing in Kirkwall apart from being nosy.
The reason why people prefer the Warden is because they were plot integral, they were the only ones capable of defeating the Archdemon and ending the Blight, and in order to do that their motive was to unite the land and stop Loghain's tyranny on Ferelden; therefore the Warden's motives and reasoning for being in the plot was clear. Where as Hawke had no effect on anything that happens around him/her; Anders still blows up the chantry, Orsino still turns to blood magic, Meredith still tries to execute all mages even though the real culprit is standing in front of her about 3 feet away.
So in my opinion the reason why people hate on Hawke is because they feel the need to blame the character for the sloppy plot on Bioware's part. Would I like to see Hawke being the main character in DA3? Absolutely but we need to have a good plot for Hawke to be involved in and therefore giving them proper motives and reasons for why they are doing this apart from "because Hawke is awesome" or for the sake of doing it just to move further into the plot.