The big irony for me is that I would have totally seen Overwatch as a story-driven, semi-open (or semi-linear) single-player FPS with a proper campaign in the veins of Borderlands 2. Instead, they opted to make it multi-player only, which to me is a big disappointment.
I don't care much about the number of players limitation (if it does end up being 6 Vs 6 for example), and I don't care that it "looks like TF2" (although it does have somewhat of a unique feel on its own as well), my 'problem' really is the fact that it's MP-only. The fact that it's MP-only put it directly in TF2's lawn (and not just because it "looks like it" but also because it's pretty much the same game-play mechanics, team and class-based combat). And, honestly, I do believe that TF2 will remain successful for years to come, whilst Overwatch might perhaps reach the Fad status at best.
I don't see it going anywhere near where Blizzard thinks it can go. I might be wrong of course, time will tell. I would like to add, however, that if there's one thing I "agree" with (or that I'm happy about) is to see Blizzard trying something new... I really don't blame them for trying something outside of their box (RTSes, and aRPGs).