Avatar is okay. I watched it once with my brother, and once on my own. The plot really didn't do it for me, and the end battle is okay... Though Camereon's script really had me rooting for Quatrich and his PMC's over the lead(Jake?). Seriously, Our Hero flips sides in a heartbeat by comparison to say... Dances With Wolves, where it takes Dunbar quite awhile before he really flips.
As to Abyss, it was in competition with two... Maybe three other very very similar movies. One of which was Leviathan, and I forget the other one, Deep Water something or other? Basically a bunch of deep sea "horror" movies all at once, and that's why Abyss didn't do as good as it could have.
I don't know how successful it was, but I've always thought True Lies would fit into the realm of "lesser Cameron." Not because it's bad, it's really quite good, but it does come right in between Terminator 2 and Titanic. Surrounded by titans.
Also, i'm still not convinced Ridley is doing a whole other trilogy. My gut really tells me that Covenant is #2, and then he might do one more that leads directly into Alien.