- Assassin: As some here have mentioned, it takes quite a bit of points to make this character useable, and sadly, many stray off from using her in the higher difficulties due to her being a bit underwhelming, as well as a low exp generator. To get 'I Was Never Here' and 'Knife in the Shadows', we are forced to go through Flank Attack and Fallback Plan. Perhaps finding a way to get them earlier would be nice, though i'm not sure how much it would help the character itself.
- Keeper: I know some people here will claim that 'Strength of Spirits' is meh, but for those who want to optimize her barriers, you have to go all the way down the middle tree to obtain it. By then, you can barely touch her right tree, because you're still trying to snag some more passives/abilities on her left. Of course, this could easily come down to a matter of playstyle, but the fact of the matter is that it takes many levels to have the Keeper ready for the higher difficulties. Also, the Virtuoso can get all his barrier/mana regeneration passives by level 9, while the Keeper is still struggling to get them at level 10+
- Alchemist: In a similar boat to the Assassin, one has a long way to go to get 'I Was Never Here' while being forced to grab 'Shadow Strike'. I know not everyone here likes using her as a stealth character, but if one does, I think, they would prefer to spend their points in other abilities. I personally would love to be able to get 'Gaps in the Armor' too, without having to go through 'Twin Fangs'.
- Arcane Warrior: I can't say much due to having used this character so little, but all I can say is, some kind of threat generation seems more fitting for this character over 'Peaceful Aura', such as the 'Biggest Threat' passive on some of the warriors. Since the Arcane wants to draw aggro to keep up his barrier, it would seem fitting for him, as well as the team. I guess you could see this as one of those passive replacements?
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I'm sure there's more that could be brought up, such as the Templar taking many points to use, but I'll just stop there for now. Feel free to critique, add suggestions, and discuss.