Beware of excessive aggression especially in cities, you can close entire quest paths by attacking (or angering) the "wrong" NPCs. The game doesn't limit what you can do, which is awesome, but everything you do can and probably will have consequences.
I once had to reload a save because the guy from who I needed to get vital information from got aggroed because he saw me stealing, well, "looting" some stuff inside his house.
Also try to get as much info as possible about a particular situation before comitting to irreversible decisions (although it's not always easy), not everything is what it may seem at first glance, and not everything NPCs tell you is always true.
It's a truly wonderful game if you enjoy this kind of games, but it requires quite a lot of time to get the most of it.