I realize that you (the developers) have tried to follow the rules of D&D closely, but there are some times where this hurts the game. The "save" system of D&D exists to control the fine details of task executions since the player simply can't do it themselves in a turn based informal environment. However, in an online MMO, a lot of those details (such as jumping out of the way of a spell, or springing back up from being knocked down) CAN reasonably be controlled, and defaulting to the old D&D system now becomes a denial of control that feels very bad to the player.
Currently, you have random rolls (luck) controlling too many of the things that players should have control over, such as recovering from crowd control and applying crowd control. Players have no clue what their chances of their spells succeeding actually are, and they often have to spam them to get them to work. This is the opposite of fun. Also, players can be locked in place by stuns, paralysis or dance effects for long periods of time with no recourse. This is also the opposite of fun.
Control is what makes games different from just watching a movie. We (the players) want our button presses to feel important. Overall, the effects of luck on crowd control need to be diminished, and here's how to do that:
1. When it comes to spells/abilities that cause crowd control effects, unless the target is obviously immune to them, they should ALWAYS take some effect, but have a duration that depends on the old DC and saves. Not all abilities have to work the same either. You could have saves cause half duration for some abilities. You could have the saves get stronger and stronger every few seconds for other abilities that allow a "new save every X seconds". You could have a base duration plus a secondary duration based on what the save probability would have been for some abilities. The point is to reduce the effects of luck.
2. CC breakers need to be a thing. There are very few of them in the game, and the ones that do exist don't even work properly. This is very unfortunate, since CC breakers (like combo breakers) are very fun for players. Rather than forcing players (and mobs) to wait and do nothing for the duration that they are stunned, give them a selection of things they can do. There are a ton of abilities already in the game that you can just put a "CC break" effect onto with ease. Give players control; control is fun.
3. Don't allow CC to spammed. Players should not be permanently locked in place, and neither should mobs. Put better limits on CC effects, such as shorter maximum duration, longer cooldowns, etc.