What are the consequences of stealing?

There is quite a lot of nice stuff just laying around, useful and valuable items that their owners are not actually making good use of. I'm currently saving the world in sub-standard equipment, so I was thinking of just taking what really should have been mine in the first place.

What are the consequences of stealing stuff in Skyrim? Are there only consequences if I'm seen stealing anything, or do I get something like "bad karma" even if nobody ever sees me take anything?

Generally, when nobody's looking, you're safe: on the other hand, stealing—and getting caught—will place a bounty on your head for half the item's value.

There aren't any permanent consequences for stealing in front of witnesses, but they will eventually report you to the authorities, which will start the bounty and alert the guards.

There is a small chance that you rob from someone who can tell when you stole something from them, even if nobody's around. The consequence of doing that is a set of hired thugs sent after you. This seems to only happen once: if you defeat the thugs, you're safe until the next high-value theft.

I've recently had a run in with some hired bandits who had a contract on them. Apparently, someone I stole stuff from hired them to attack me and try to kill me as revenge. I'm pretty sure no one saw me stealing, but I guess they figured it out somehow.

So there are some consequences.

If you really intentionally do a lot of bad to a city, and they see it all, you will...

1. ...get a bounty, high enough so you can't really pay it anytime soon.

2. ...will be followed / found in the nearby areas of the city, if you use the wait system you are suddenly surrounded by an angry group of guards and persons that have passed by. Some of these might have been hired to do, these type usually travel farther than the average person.

3. ...upon paying/losing the bounty or going to jail; when you return to your city, they will be funny and will be for example searching for a thief with your name but the opposite gender of your character, and they will be complaining about the security/guards of the city not finding the thief.

   This last thing, are only scripted dialogs and have no further effect on your standing.

If you aren't detected while stealing, or your chances are lucky, they won't even know you stole things.