I think the biggest issue was the offensive line. In the handfull of games I saw, the quarterbacks were running for they're lives, and Mcghaee wasn't getting any room to run.
You know, and it's funny that you mention that because Bledsoe and Henry were solid, and rather than work on the OL, they draft McGahee, then dump Henry, then draft Losman and dump Bledsoe.Because of their terrible OL, they really aren't in a position to succeed for "fantasy stardom".
Here's some interesting numbers:
2002 - 8-8
2003 - 6-10
2004 - 9-7
2005 - 5-11
2003 - 58.2% completion, 2860 yards, 11TD/12INT, 73.0QBRAT
2004 - 56.9% completion, 2932 yards, 20TD/16INT, 76.6QBRAT
Losman (2005 adjusted):
2005 - 49.6% completion, 2522 yards, 15TD/15INT, 64.9 QBRAT
2002 - 325 att, 4.4ypc, 1438 yards, 13TD; 43 rec, 309 yards, TD; 11FUM, 8FL; 6 100+ games
2003 (15G) - 331 att, 4.1ypc, 1356 yards, 10TD; 28 rec, 158 yards, TD; 7FUM, 3FL; 5 100+ games
McGahee (2004 adjusted):
2004 - 379 att, 4.0ypc, 1504 yards, 17TD; 29 rec, 225 yards; 6FUM, 3FL; 9 100+ games
2005 (15G) - 325 att, 3.8ypc, 1247 yards, 5TD; 28 rec, 178 yards; 1FUM, 1FL; 5 100+ games
Clearly the Bills haven't gotten any better from their recent first round picks (I left Evans out because of the 3-yr WR rule, and it's only his 2nd, and he's not a WR1 until next year).
In fact, I'd go so far as to say they've regressed a bit. See, this is what happens when you go for QB, RB etc rather than OT, OG. The Bills still have a terrible OL now, still have (for the sake of argument), about the same production, except they've wasted two first-round picks to do it.
To answer your question, no they aren't on the verge of fantasy stardom. Evans is probably going to have a decent year next year (1100 / 6 judging from Moulds' dropoff after Price left, and then a couple other young WR1s without reliable compliments in Andre Johnson and Michael Clayton), but other than that, it's just going to be the same thing until they fix their real needs and stop wasting draft picks on upgrading positions that needn't be upgraded.
Even with a little offensive line help, who knows? Maybe, but past history indicates they would rather draft Jay Cutler or DeAngelo Williams and go 6-10.