It is fun playing with and killing iconic marvel heroes...
That is pretty much the long and short of it. The issue was always going to be 'can we make cosmetic upgrades?' and the answer is basically 'no', which is actually a big turn-off for a lot of ARPG-lootwhore players. I mean, in other Marvel games you can unlock other versions of a character's costume (Avengers, AoU, classic, etc), but that's about it.
City of Heroes faced the same issue, which is why it had easily the most robust costume creation process I've seen in an MMO. Once you got your hero's 'look', that was IT.
So the lack of visual upgrade is another thing that deters me a bit.
In Marvel Heroes each of the umpteen heroes also has a variety of costumes, as you say, classic, AoU, etc etc. However, I don't think you can individually upgrade each of those costumes. I'd suggest you give it a shot for an hour or two - gives you a decent sense of the gameplay at least.
The gameplay seems pretty fluid, it feels more like D3 than PoE - and you tend to use more buttons actively than either of those two games.
Anyway, hit me up on steam if you want to give it a whirl with me.