There were a lot more abilities in ME1 but most generally went unused in favor of things that worked basically everytime. This time around they at least try to switch things for the player up by putting different barriers on different enemies, which basically requires you to vary up what you throw at them, at least on higher difficulties. There are fewer abilities but they feel more meaningful. I would have loved to see more, as I do agree that four is a bit too few, but as you noted they could have ran into more balance issues.
I also think that outside of that, the feel of the gunplay itself is vastly superior to ME1, as well as Shepard's movement around the battlefield (though that still has its issues). I didn't look forward to the combat in Mass Effect 1, but I definitely do in Mass Effect 2. One of the main reasons I want to find anomalies is so I can get into more fights.
Also, as far as your last point... the cooldowns are short but most biotic abilities can't be used until you've breached the enemy's defenses, which on the higher difficulties is pretty much everyone you face.
