I'm with [doesn't]Needsize, Debaser and Warpath. There are definitely genetic limitations...otherwise, why would someone like Ronnie Coleman, who fights for ever-greater deadlift poundages (etc.) routinely compete around the same bodyweight? Lack of food or test ain't the answer, that's for sure.
Why can't I be a 5'6", 340 lbs. ripped bodybuilder for that matter? God knows I bust my ass and train pretty intelligently.
Answer: Ronnie's pushing the envelope of his genetics. So, too, would I be as I approached 340 ripped

(J/k...I don't even wish for that. I have enough trouble getting around 100 lbs. lighter and far from shredded. 300+ is almost scary to me.)
Where I agree with everyone is that "genetic limitations" is too often a cheesy cop-out. As much as I like most of what someone like Brooks Kubik or Stuart McRobert says, I wince every time I see the term "hard-gainer" or read where someone believes they're at their genetic limit when they don't even have 16" arms.
Generally speaking, Pragmatism Is Good (tm). Bust your tail and make the best of what you have...don't fret about limits until they're beating down your door, and fight them even then.