Naturally I'm biased, but I liked the /temporary/ exclusivity that SS2 had, and hope that titles like Steam's Duke 3D expansions, AoE2 original, X-Com, etc. eventually come here.
I would also like to see AoE2 here, but I think that is the least unlikely scenario of any theoretical scenarios. Both MS and Steam share the same ideals in terms of always-on activation. With AoE2 specifically, it seems most of the "updates" were in online type options that can help make the always-on argument.
I've learned to never say never... but I think we may well see SS1, Nolf, Grim Fandango, and even Morrowind here before we get AoE2 (hopefully, I'm wrong... actually, no, hopefully, I"m right, we need those games! ;) )