I'm against moddable online game, i don't like the risk mods turns into cheat stuff.
I never really cared for achievements, especially steam achievement. In-game achievement are just a time killer for me when I got nothing better to do. Unless there's a badass skin to unlock as I'm a skin collector (also a pay2skin).
I'm for client AND steam integration.
Steam, even if they need a fee or whatever, is also good to get the game around the world. Better commercial crap and stuff.
Rpg like customization, dunno. If you mean something like class, yes and no.
No to fixed classes, yes to "make your own class". It will be in the game, but not straight on release. Stuff like that gonna acquire a lot of resources.