He's simply one of the most unremarkable actors I've ever seen, and it seems like they are trying to make him this big, breakout marquee name. I mean, Paramount, with putting him in various vehicles is trying to do the same thing with Chris Pine, and I'd argue he has yet to breakout in his own right, but he has natural charm and honest talent, so I think it's only a matter of time. Courtney is just...there... in everything he does. Teaching a parrot to read his lines would be more entertaining. I think it boils down to the simple fact that Marvel has yet to lose sight of the fact that they make movies based on comic books. They,even when going to serious places as with Winter Soldier and Civil War, seem to grasp that the viewer wants some high-energy escapism. DC is mired in this almost morose, Gothic tone that's just off-putting. They think they're deep and they are soooooooo not.
IMO, the worst thing about TMP is that it isn't a Star Trek movie. Just as you say above: The characters are bland and interchangeable, they're not the same as from TOS. That is my biggest complaint. Which is a pity, because it's really not a bad movie. "2001 for dummies" hits the nail on the head, IMO -- including some of the greatness in scope of 2001. But I feel it doesn't age as well as 2001, and it is supposedly much less impressive on the small screen. So IMO, perhaps the best of the worst ST movies. TFF suffers from the opposite problem: It has very nice character moments that make it worthwhile rewatching for a true ST fan, but it's a very bad movie. (And INS and NEM are just awful on many levels.)
^ This. And yes, I too have a soft spot for TMP as well; it was the ONLY movie in the ST canon (still) to ever make me feel the awe, mystery and scale of space travel. None of the other ST movies have ever really done that, really. They're more episodic; like the TV series. But TMP (as its anachronistic title implies) truly is Star Trek as a MOTION PICTURE. And yes, the characters were riding shotgun to the experience; they were essentially tour guides. Familiar faces to guide us on this extraordinary trip. It was a unique experiment in ST; very ambitious, and on some purely aesthetic level it succeeded (IMO, at least...). "The Human Adventure Is Just Beginning..."