There are many ways a movie can be terrible, completely independent of the way a Star Wars movie can be terrible, and we should probably get those out of the way first. Movies can have nonsensical plots, or tedious pacing, or thinly drawn characters, or wooden actors, or any of a few dozen pedestrian flaws like that.
I will grant that there’s reason to be optimistic on this front. The director, J.J. Abrams, has made many movies that don’t suck, most notably the pretty successful reboot of the Star Trek franchise. All of the original cast (aside from Alec Guinness, who did not wish to participate) will be back, playing the characters we know and love, and sure, some were a little wooden before, but it was a woodenness we came to accept.
But it’s worth pointing out that pretty good directors and pretty good actors can still make bad movies. It happens a few hundred times a year, in fact. That might be the saddest possible outcome of all this: that the new Star Wars isn’t great, nor horrible in an interesting way. It could just be mediocre.