First thing I did on the beta was co-op with a friend. It actually felt a lot more challenging than with a full-party. It was the only time I died all weekend. In fact, I think that was the only character I saw die all weekend! Granted we're only seeing a snippet of the first act but as part of a four-person party, it seemed very difficult to fail.
Not sure about any classes being over/underpowered. A lot seemed to rest on scavenging a decent weapon. Witch Doctor I felt had some of the least satisfactory skills and felt like the weakest class of all the characters I tried, but I ended up hitting my highest DPS with him! With the Monk I had high expectations, but faded later on as I couldn't find a good enough weapon.
I think I must have spent too much time doing runs. I only encountered the Jar of Souls event once, arriving halfway through it. Will have to do some more thorough exploration when the game comes out proper! >.<
