You walk around crushing everything. It's no challenge. Then all of a sudden you're dead, and you literally don't know what hit you. It irks me so much!

That's because you become careless and stop paying attention. Then, BOOM, suddenly you have 3 Elite monsters on you and you are dead....

Happened to me so many times :)

And I guess you are playing on Normal or Elite. On Legendary, you have to always pay attention unless you are geared in all purples.