As one of the people who bashed Eragon (and has no problem using the word "sucks" to describe it), let me elaborate on why, I feel, it sucks.
The writing is inconsistent: Remember when he first gets the spiffy sword? To paraphrase, "It felt like an extension of his arm." Later, when he actually has to fight, the sword isn't like an extension of his arm, "it's as unwieldly as a club." What? Well is the sword balanced and easy to use or not?!? If the guy who wrote the book had said when the kid first got the sword that it was "surprisingly light and well-balanced, but without any training, it was as useful as a club to Eragon" or something along those lines, it would've actually made sense. But instead, as a reader, I had to jump back in the book to confirm the glaring contradiction. I think this is an example of objectively bad writing: you can't say one thing and then say the opposite just because, "Oh yeah, I'm gonna put in some cool sword-training parts with the Obi-Wan guy in the next few chapters, so Eragon should have a reason not to use the really cool sword that's like an extension of his arm right now. Hmm, now it'll be useless like a club, yeah, that'll work!"
The Eragon character also evokes the wrong emotions from me: I gave up on the book around the 30th time Eragon cried like a baby (yes, I'm exaggerating, but man did that boy cry a lot). Maybe other readers felt sorry for him and thus became more fond of the character and the book, but I wanted him to grow a pair and shut the hell up. Unsypmathetic on my part? Absolutely. Subjective? Yeah, probably. I also found Sansa Stark from George R.R. Martin's books (which I love) to be insufferable (although in the latest one she seems to be growing a spine, which is a nice development).
So why do I love A Song of Ice and Fire and hate Eragon? Partly because of the authors' decisions about who those characters (Sansa and Eragon) are supposed to be. It's okay for me to find Sansa Stark to be a useless twit who causes untold ruin for her family...she's supposed to be a useless twit who causes untold ruin for her family. Eragon on the other hand is supposed to be the first person in decades capable of freeing the kingdom from Captain Evil...crying every ten minutes doesn't really fit with that. Now, my preferences are more complex than this (Martin's writing has a maturity that "the kid" simply doesn't have, his sentences are tighter, his descriptions more vivid, his characters are far more three-dimensional, and his plots actually surprise you while still making sense when considering previous events of the storyline), but hopefully I've given you some idea of why I feel the way I do.