What are the consequences of stealing in this game? I just had a group of hired thugs attack me because I stole some potions from a temple (without getting caught). Are there any more serious consequences for stealing even if you don't get caught?

Sometimes you get the Dark Brotherhood sent after you.

I guess what I'm asking is if there are any drawbacks quest wise, like will people stop talking to me or selling me their wares?

People won't buy your stolen goods. If you're caught, you obviously get a bounty on you. If your bounty is high enough, people will simply attack you on sight and not bother with trying to arrest you, even if you yield.

Even if you're not caught, people can sent the Dark Brotherhood or other Hired Thugs after you.

It doesn't affect the story one way or another, though.

If you pickpocket someone from the story they will most likely ignore you, or say something like "You could've just asked"

Which is funny, because the only person I've spoken to that you literally can ask is Alvor. He has the dialogue option of, "Anything you can give to help me?" and it will open an inventory screen with everything he has on him or in his house that he will allow you to take.

Also if you get caught by the guards, they will take all your stolen goods. I have some pretty decent gear from pickpocketing..it wouldn't be fun if the guards catch me now!

I like it that some people know you stole from them despite you not getting caught; it gives the impression of intelligence. Anyone with a brain could figure out that if your goods in the back room went missing after a shady character went in there, it was most likely them who took it.