I remember a first surprise being the Circle of Thorns demon portals. I was on a team, and we were trying to find a glowie or something. They found it, but I was enthralled by these portals that just kept spawning demons non-stop. Someone who lived in my dorm happened to come in, and called me a "terrible teammate" because the rest of the team was off fighting objective bad guys, while I was over by the portal spawning endless masses of demons. Oops?
Regarding scrapper lock: Dual Blades scrappers actually have a reason to move from group to group - if the finished one group on the first or second step of a combo, there's a limited time to get to the next group and finish it off.