Although i haven't played the previous Oblivion series games, i find Elder Scrolls IV the best single player RPG game I've ever played. The class system is extremely flexible and if you have done the newbie tutorial once, you won't have to do it again if you want to create a new character (save right before the final gate). I'm not a basher-kill person although i DO sometimes randomly kill things for fun =P.
I like the realistic atmosphere presented in Oblivion. Whenever i go out on my stealing trips, if i don't get caught, the item's mine for the keeping

. Once i even ransacked an entire barracks full of weapons and armory till i went over my weight limit 3 times (had to take about 3-4 more trips of stealing-fencing, before finally ransacking the whole place). Basically, the only thing you need as a thief is to have sharp reflexes, good eyesight and good hearing. Boost your speakers to its maximum if you're playing seriouslyl. If you don't listen intently enough, someone might sneak up behind you.
What i don't seem to understand is that after you steal something from a person and he/she doesn't find out. They don't seem troubled or to mind it at all. I once went on a key-stealing frenzy and stole the keys to at least 5 different houses in Tamriel (is that what the main city is called?). Nobody noticed

.
Still, if you're looking for a good addictive game to burn time, get Oblivion.