I think another way to look at it though is that the first game was more of the start up of a grand space opera; and as I said in my wall of text above, more immediate or urgent. So yes, I think as a self-contained story as much as an act can be, it did it the best by far.
However, the second wasn't really a slouch in the writing aspects either, it simply changed the format to more of an episodic nature. Instead of one grand chapter, it was broken up into many more bite-sized chapters that didn't have to be played in an entirely sequential order. I truly believe that the second had much more emotional involvement than the first, whereas the first had more awe factor, especially because everything was fresh.
Yeah, if you end up enjoying the second, you probably should pick up the third just to see the story completed. I dunno about the combat not bothering someone though. I typically don't play FPS/TPS (been there, done that from the 90s on up) and I absolutely loved the combat in the second aside from the lame squad AI (especially on Insanity). The actual combat mechanics may have been improved in the third, but it definitely doesn't feel the same as the second. The second had a ton of action, but it wasn't so overbearing as it is in the third--where it just feels like it's non stop, and the way the enemies would move sometimes was borderline comical. Bioware claimed to have massively improved the AI in the third, but all they really did was give it a very minor tweak and threw twice as many enemies at you hoping that you wouldn't notice.