1.) Having the same man in charge of Star Wars and Star Trek creates a conflict of interest. This is like hiring the same marketing director for Pepsi and Coke. This is like employing the same head chef for the test kitchens of Burger King and McDonald's. This is like...I dunno, casting Ryan Reynolds as Deadpool and Hal Jordan.
2.) I think I've made my opinion of J.J. Abrams pretty darn clear. Those are hyperlinks, kids; don't be shy to click 'em.
3.) Star Trek (2009) was pretty much a Star Wars movie anyhow, so this should be a perfect fit.
I don't have it in me to really discuss this further. J.J. Abrams just makes me angry, and there is no one in Hollywood who makes me angry just by seeing their name attached to something. Disappointed, sure; uninterested in seeing the movie, sure. Angry? No. He's officially become the face of sci-fi for "the modern generation," and from what I've seen of his work, that means I can look forward to a few decades of movies so flashy that they almost make you forget how contrived the script is, how abrasive and unlikable the characters are, and how little the heart and style of a franchise that's not your own really matter.
But you know what? It's not just J.J. Abrams. It's the likes of Roberto Orci and Alex Kurtzman, too. You know—the guys who co-wrote and/or produced The Legend of Zorro, Transformers, Cowboys and Aliens, and Star Trek? I'm sure you remember those movies—fancy action sequences, major plot points that get minimal or no explanation, comedic digressions that completely break the flow of the movie, characters who aren't so much complex heroes as walking personalities that save the day when the story calls for it...
The fact that Orci and Kurtzman did Watchmen, and I liked Watchmen, would seem to throw off the curve here. Except I didn't care much for the source material (respected it, but didn't like it—too dark and gritty for my tastes, for one thing), and the movie was a direct translation the graphic novel, minus almost everything I didn't like about it. In other words, the script was already written for them, and they just edited out some parts I wasn't so keen on; I already knew the story, so I could fill in any plot holes with what I remembered from the book; and the characters in Watchmen are supposed to be less-than-ideal heroes.
So maybe I'm not angry that J.J. Abrams is directing Star Wars. Maybe I'm angry that Star Wars—like Watchmen, for Orci and Kurtzman—is the kind of thing he should've been doing all along. And I can't help but think that someone else would've been tapped to direct Star Trek if Star Wars had gotten to Abrams first.