One problem I've noticed is that the heal potions are not powerful enough for high level play. They have not kept up with the massive amount of damage players are expected to withstand at endgame levels. It is not unusual for a player to be fighting one of the godlike Humania yetis and be unable to heal as fast as its aoe blasts them down, even spamming potions continuously. The problem is that you can only use one potion per second (or so), so you can pound the button non-stop and still lose the damage race with the all-powerful one-shot machines that currently populate the endgame levels.
One way to fix it would be to remove the delay, allowing players to use potions as fast as they can push the button. A better solution though would be to raise the strength of the potions proportionally to the player's level. This would mean potions would also have to go up in price at higher levels, but that would be easy to justify since players wouldn't have to use as many to stay alive.
This is a necessary change as long as you guys continue down the path of inserting insane aoe damage outputting mobs in the game. There's nothing more depressing than knowing the team-wiping shot is coming, pounding the heal potions for all you're worth and still going down. There's literally nothing else you can do and you still lose. That's dumb. Give us strong enough potions so that players who learn when to brace for the death strike at least have a chance of surviving it. Or you could, you know, design monsters that aren't unstoppable one-shot machines, but that's crazy talk.