Off the top of my head, the only three people who have just up and left and never come back are Michelle, Hitomi, & Tsuyoshi. It was bound to happen with Hitomi & Michelle, given their personalities, and Yoshi can pretty much still be seen drawing awesomeness on the dA. What these three have in common was that the just up & left without really saying a word. Those are the people I never expect to come back.
Oh, and Asvel.
Actually I do remember Yoshi saying he was leaving and never returning, don't remember if Hitomi did too, but knowing her, if she did it would've just been a quick mention if anything at all, so it's more like they just said a quick good bye, didn't make a big deal out of it and just left. Michelle just disappeared before I joined, and god knows what happened to Asvel, did he ever post anything after that last comic jam of his?
Oh, and shame on you people, the sheep god (and with him Alma) left recently and you don't remember >.>
So, of the top of my head, Michelle, Asvel, and Nobo left without saying anything, while Yoshi, and Ares said their good byes and kept their word. Hitomi as I said I don't remember >.>