Firstly, while "DEADPOOL" was an entertaining action-packed romp, it likely didn't help that it was a story taking place in Ultimate Spider-Man rather than Ultimate X-Men. Deadpool had to be made to be an opposing match to Spidey while still fulfilling his role to be the main foil for Bendis' sequal to Vaughan's "LONGSHOT" arc. Despite the fact that Spidey being involved with the X-Men makes more sense now that he's dating Kitty, it still had an "Ultimate Marvel Team-Up" feel to it (as did far worse stories during the SUPERSTARS run). Obviously Bendis misses that title, but it was the poorest selling of the Ultimate line (barring ULTIMATE ADVENTURES, of course. I doubt Hawk-Owl's being seen again).
Secondly, Ultimate Deadpool was played "straighter" than his 616 version has become. 616 Deadpool has almost become a more lethal version of Freakazoid, whereas Ultimate Deadpool was more of a mercenary warrior with a dry wit. The downside of this is that "gunman with a quick tongue" are pretty much a dime a dozen, especially if you consider manga and anime. Vash the Stampede, Monkey C. Luffy and Lupin the Third have fought dozens like Deadpool.
That all said, I liked "DEADPOOL", but that's probably because I never really cared for 616 Deadpool much, only seeing him as a bare minimum character (a "more twisted Spider-Man with a healing factor" basically), and therefore wasn't disappointed in his presentation. If anything disappointed me about DEADPOOL, which I did like, it was that Spider-Man has become Eric Foreman (of THAT 70'S SHOW); someone who needs his girlfriend to fight and win his battles for him. One minute Kitty's a ditzy teenager, the next she's a flawless X-trained warrior, and she needs to pick one or the other, otherwise predicting her "saving the day" will become as old as predicting the next time Spider-Man will be defeated and unmasked. USM would be a flawless title if Bendis could learn to think outside his own box sometimes.