They aren't so bad. The biggest downside is the way the character swapping is handled and in addition to that, bank space. You have very limited space to store items. The bank slots are probably 6 by 8. You can store some stuff in your "Team Up/Sidekick" inventory even if you don't have a character unlocked for that which is maybe another 20-30 items.
There's no traditional character menu like in Diablo or WoW. You pick your starting hero and that is permanent on your account (tied to an email address). If you want to play another hero immediately, you'd have to create a throwaway email address and then another account. It does take time to unlock the other characters, less time if you aren't choosy (Random Hero Unlock is 175 tokens, picking a specific Hero can be 200, 400 or 600). The drop rate on those tokens is about one every eight minutes but you get them from gifts and bonuses like completing the story or logging in X amount of days.
When you switch character, your current character "calls them in" and that's the only thing that is swapped; your inventory is shared between all Heros. Say you had a Medkit in slot one on Hero 1. When you call in Hero 2, you have that same Medkit in slot one. Same goes for the bank, new characters don't net you more storage space.
So far the storage space looks like you'd have to buy it with the Shop purchased currency, you do get some as a story or a login bonus (I don't recall which) but it isn't enough to unlock another bank tab.
An interesting feature is the vendors can be leveled like in D3. Instead of selling items to them you can choose to donate them and they gain experience based on the value of the item. This can be a bit of a grind at times but thankfully the Midtown Patrol event in game gives you plenty of items to donate. As they reach new levels, you get access to more abilities: you can combine higher level enhancements (gems essentially), buy more powerful items and apply better bonuses to your gear.
Lag can be an issue at times when you are in open areas and there are lots of heroes on screen. I keep my graphics settings turned down for this but it is a server processing issue as well so the slideshow can only be avoided somewhat. Other than those issues I haven't run into any other negatives yet but I still don't have a max level character.