Cassel never quite developed beyond being a capable, but slightly below average, NFL starting quarterback. He piled up 59 touchdowns and 44 interceptions in four years with the Chiefs and helped the team win the AFC West title in 2010. But his accuracy, pocket composure and ability to improve the players around him never developed.
He was a solid-enough option in 2009, but the Chiefs surely hoped that he'd improve more with additional time on the field. Cassel has been closely tied in rumors to the Minnesota Vikings as a potential backup to Christian Ponder. Cassel could also fit in Arizona as a potential starter, or back in New England as a backup quarterback if the Patriots trade Ryan Mallet.
Cassel isn't quite a "franchise quarterback" but he should improve some team fairly soon. Look for him to find a job.