I just finished the campaign and I gotta post my impressions. Overall, it was really good, by far the best RTS campaign I've ever played... But the story was bad. Heavy spoilers follow, so beware.
So I didn't really dislike the ending at all, I could have totally accepted it, I also could totally accept the cliche'd dialogs and shit (because the lord knows it got really corny at some points, but I could have totally buy into them), but my main problem with the storyline is that the plot and every element of the story is just too thin, nothing much really happens, and everything that happens is very straightforward, there aren't many plot twists or surpises, and when the plot developed, we were just like "well, ok, I guess". Let me exemplify, climax of the story is presented before the last 3 missions and it goes "well we will invade Char and try to turn Kerrigan into a human again", and that's exactly what happens, there is nothing more to it, and we begin to feel that something is missing. The major plot development in the entire game is that you side with Mengsks son, and that happens in one cutscene with a few lines of dialogue. And we could totally accept that, but we can't help but ask ourselves, "was that it?". The highlight of the story was the Zeratul stuff, and while it did present something (even if its optional), I don't think it redeems everything or makes the overall plot not suck as much.
This is perhaps best seen in the whole Tychus thing, we know he is working for someone else, this is implied many times in the story, but when it happens, in what could have been the major plot twist of the story, it just goes exactly as we expected it to, and all the build up could be resolved with a simple motive and 15 seconds of a cutscene.
In sum, the story had the epicness, it had the presentation, the cinematics, the room for dialogues, etc. But the story was so straightforward that it feels out of place. I reached the conclusion that the first SC didn't feel like this despite having a similar simplistic nature, its because the presentation was much more sober, it was much more subtle so the thin story had to do more in a more limited fashion, and this made it good. With SC2 you had the all the elements to make an amazing story, but it wasn't there.
The second thing that bothers me is that it felt incomplete, and this is more obvious and has been more brought out. I'm not talking about the cliffhanger ending, which is understandable. But considering we will not have any focus on the Terran side again, except in the form of cameos, makes it all the more disappointing. I bet we won't see the fight between Raynor and Mengsk, we won't see directly what goes on between Mengsk and his son, or with the raynor fight, and since we had barely any exposition at all in this first installment, it left me with a bittersweet taste in my mouth.
