I lived in Boston last football season and got every Patriots game on tv so I feel I can fairly critique Matt Cassel. He does not have the physical tools that Cutler or Orton have. He was simply put behind the best O-Line in the league with arguably the best receiver in the league in Moss and the playmaker Wes Welker. I think he can be great if he is in a great situation, but he won't single-handedly win any games. He doesn't have an extremely strong arm, has suspect accuracy when under pressure(more than most) and he locks on to receivers. He doesn't make adjustments at the line well and won't pick a team apart unless given ages to throw as he was this past year.
I don't think he is any better physically than Shaun Hill having watched both of them this past year and I think he still has to develop a little bit. He will, however, be a solid solid starter for the Chiefs.
Having said that, ex-Pats GM Piolli traded for him and the ex-Pats OC WANTED to trade for him so there must be something I'm missing.