I love all the characters, they bring such different options to the game. Cat and dog the simpler of them, as they were there at launch. Think I'm sitting at 125 hours or so in this. Would be more but one of my friend's isn't a fan... he says it puts him to sleep, which just means he's not running and jumping around enough.
Swords is probably the one addition that didn't ever really click with me well, any characters good at close quarters I'd rather just have a shotgun on. There arae still some decent swords though.
I really like the progression, also. Unlocking things for killing x number of a certain creature, killing some bosses for the first time, etc.
Also, for shard farming upping the difficult is good too, though not sure you can until you beat the game once.