You and naturalethic should check out the #weeklymusicthread. I can tell you two would dig them.
You know, I thought about this some more and I think a better word than mellow is 'somber'.
Architecture in Helsinki is one of the few on your list that I like actually. My favorite album right now is YACHT - Shangri-La. I suppose I prefer stuff that has a strong bass component somewhere, and a clear well-defined beat. If has some other element that is otherwise just real interesting that can mitigate a lack of other factors. Alt-J, for example, has just such awesome melody and vocals that they can lack on other things. I also really steer toward music that has an 'epicness' to it. I'm not sure how to define that, and may be fairly subjective.