I focus on the negatives, too, but give Beane credit for giving us, in no particular order, Oliver, Bishop, Walker, Hoecht, Bosa, Thompson, Hairston, Kincaid, Hawes, Shakir, Cook, Benford, the entire offensive line, and of course number 17. Most teams would snap these players up in a heartbeat. He's also provided quality role players, filled holes deftly (Prater, Wishnowsky), and, as GunnerBill pointed out, over time built a deep roster, with players like Trubinsky, Knox, Ty Johnson, VDM, Hancock, and the many recycled oldsters on the team. Picking very late in the draft every year (late on the waiver wire doesn't help, either) it's not surprising he's found it hard to find impact players, but he has done an admirable job filling out the roster, overall.
The glaring, neon-blinking, jaw-dropping, almost unforgiveable exception of course is wide receiver. That's the big hole on his resume. If he turned out right on either Coleman or Palmer, we would be the Super Bowl favorite, hands down. But he wasn't right. He could not have been more wrong. It's not much of an exaggeration to say he is asking Allen to play with a nice line, quality RBs and TEs--and NO wide receivers at all, aside from Shakir, who is like a little 12-year old asked to support his five younger siblings.
Put it this way: The Bills might have the worst set of wide receivers in the league--and still be the best team. That's both a testament to Beane's work as GM and an indictment.