I like Thanatos. I mean I don't like every story he was in ... pretty much just the first one. Anyway ostensibly he's just yet another Demon from some form of Hell that does the whole "I'm your evil reflection" gimmick to Aquaman, and it's cool that historically Aquaman has dealt with these fifth-dimensional sphere of the gods beings like Thanatos and Quisp and Poseidon and has some experience in the weirdness (after all, he fights Lovecraft monsters and space travels a fair bit for a pulp adventures ocean guy).
What interests me is that Thanatos is a type of villain a lot of other heroes have - the demon posing as you. Batman has like Doctor Hurt. Kyle Rayner has Oblivion. And on and on.