The only way they'd ever succeeded in making super-powerful Superman "work" in the context of a half-hour team cartoon was to make him REALLY stupid. This scatologically funny site describes it best:
http://www.seanbaby.com/superfriends/superman.htmI didn't mind having him toned down a little. Turning back time to end a blockbuster movie is one thing. Turning back time (or some similar act of deus ex Supermachina) to end every other cartoon episode can make the whole Superman experience ho-hum. It's hard to add suspense to a guy who, in pre-Crisis glory, could solve most of the world's problems before breakfast and have time to pitch at that charity softball game.
I did mind when an electric manhole cover was enough to stun Superman, or when we had to wear a stupid space suit and constantly relied on other scientists rather than invoke science on their own. They erred on the side of making him a wimp a little too often, but they improved on that over time. I decidedly -didn't- like how JLU ended, with Superman giving a big speech about how the world is like cardboard, doing some thunderous punches, but ultimately getting his butt kicked.
Mercy Graves was pretty awesome. Lex needed an entourage, and she's a more interesting villain than, say, Dumbsday. I think it's neat that her voice actor has had the best post-S:TAS career of any of the DCAU, basically in an interesting power relationship with a mad scientist sort.