Monte is a professional, and an acclaimed designer in many circles, and I expect he'll give an honest effort producing a game that is fun. The whole team will. However, I get the feeling he has very little (or maybe no) play or design experience with 4e from his L&L, journal, interviews, and twitter. That really does bum me out, given the many positives I feel have come from this current iteration.
I don't know, maybe that's what the rpg team is looking for, fresh perspective accompanying 4e designers. Design is definitely a group effort, and it seems like they're trying to take the best of every edition into account, but I'm kind of turned off by his joining, and especially his published ruminations on the site, while 4e is still living. I guess I'm also a little sore about Baker leaving, who was doing positive things with 4e, and Monte joining, waxing poetic about things that, while interesting, don't pertain to the current edition, my edition.
Maybe if he actually dipped a toe into the current edition, tried to co-design something, or just acknowledged it, I'd feel a little better. Instead he's kind of just there, writing stuff, working behind the scenes, and I don't like it.