WordBallon points out that Marvel hasn't created any new X-Men characters and it's not based on laziness. The comic book writer/creator is responsible for giving birth to Cable, Deadpool, Domino, Feral and X-Force. So when he talks X-Men he does have insider information. He's also been in active talks with 20th Century Fox over the Deadpool movie and possibly adding Cable to the X-Men films.
Liefeld states Marvel/Disney isn't in the business of creating characters only to hand them over to 20th Century Fox for new additions to their films. Currently the studio holds the film rights to all the characters in the X-Men universe (hero and villains) which is an extensive group. I'm not sure if that also means that Fox gains rights to new characters as they are introduced in the comics. It's further pointed out that Marvel holds the toy rights to X-Men First Class but didn't release any over the summer when it was released. Since they would have competed with Captain America and Thor toys which Marvel gets a bigger profit from. He's pointing out Marvel is undercutting Fox's growth of their X-Men franchise where they can.
My take is that Marvel is smart by not helping Fox by handing over new characters bound by an old rights deal. They could feel the deal was unfair since it was drawn up during Marvel's financial troubles during the 90's and might not have predicted the future of the company. Hopefully there will be a way to wiggle the film rights back to Marvel other than cutting a billion dollar cheque which Rob suggests. I feel when the studio uses up all the villains they'll end up selling the rights back. The X-Men universe doesn't have a lot of film-friendly villains which must be annoying for Fox.