Super Sidekicking -- nice! You can join any team without having to dick around with levels and sidekicks and stuff. We played with some of Rowyn's friends for a bit, until they went to bed. Of course, to see this you need a team and I've forgotten how to build teams on my own, if I ever knew. v.v
New Difficulty Settings -- eh. It's not '+4 players on the team', it's 'at least 4 players on the team'. So solo they work how you'd expect, but to get something like the old unyielding you have to manually adjust your difficulty whenever someone joins the team -- and you can't get it at all if you're on a team of 8. The 'levels above me' part works fine, but I prefer fighting swarms of even-level folks on most characters.
But unfortuantely, CO is still shiny enough to distract me, so I guess it'll be a while until I fully explore I16. OTOH the gamespot review of CO says that you should be able to finish all the content in a single month, which does seem sort of likely. I'm still getting 2 levels a session in the 20s (out of 40).