the Chiefs are lucky they only had to give up a second round pick (#34 overall) to get Cassel (and also got Vrabel as part of the trade). the Broncos had a deal set up that would send Cutler to Denver as part of a three-team trade with the Bucs. New England would have received Denver's first round pick (#12 overall), Cutler would have gone to Tampa, and Tampa would have sent first and third rounders to Denver.
The franchise tag makes this trade unappealing for KC. While I think Cassel is an upgrade over Thigpen, Thigpen played very well down the stretch last season, and should have been given a legitimate opportunity to run the team this year (given what KC had to give up for Cassel). Also, KC did not have an extension worked out with Cassel before trading for him, so now Cassel has some leverage. I would have only done the trade had Cassel agreed to an extension that paid him peanuts for the first year or two (to offset the $14.whatever million he's getting this year)... or if they could renegotiate the $14 million number down by like $5 million in exchange for a big extension... but since Cassel signed the tender, I don't think there's anything anyone can do to reduce the $14 million he's getting this season.