Is there a way to auto-attack the nearest enemy until there are no more enemies in the vicinity?
The first Torchlight had an annoying combat issue that required constant clicking/holding on the enemy you wanted to target. After the enemy was dead, you had to click/hold on a new target.
Torchlight 2 seems to have improved on this a bit. Click/hold on an enemy to attack and kill it just the same. However, while you are holding, you can hover the cursor over a new enemy. When the enemy you are attacking is dead, you will immediately attack the second enemy provided you were able to keep the cursor directly over it.
That's not good enough, though, as I still find myself losing my cursor in the thick of all the action, especially when there are huge mobs. If I could just click and hold and have my character just auto-attack everything in sight until everything is dead, I would like that very much.
While there is no auto-aim, you could use SHIFT+attack to stay in place and rotate your mouse to have your character face the way of the mobs. It will be a lot easier to do this with a ranged class as it will fire its projectiles the way it's facing.
Note that this is also possible to SHIFT+attack with a melee class as it will swing is weapon in the air, but you'll have problem with ranged mobs.
Also, as pointed out by Miki Watts, using CTRL+attack will have your character only attack or move if there is a valid target under the mouse cursor. So, it can be usefull to combine CTRL+SHIFT+attack to prevent mana lost if you ever lose sight of your mouse cursor.
Otherwise, there is no auto-win button that will just have your character switch to the nearest target automaticaly.