Oh boy. Apparently it's not a game at these levels -- it's just a mess.
Most of my knowledge of the game systems comes from Din's Curse. Even back then, cast time was obnoxiously strong and the only way to run a magic build (if I remember correctly -- it's been a while). It looks like some stuff has changed since then to make magic builds more powerful at higher levels. DL has the added issue of essentially giving you the ability to pick any skill at will (given how many mutation points there are, and with no further limits), thus making every skill compete with every other skill in any tree, so that AOE skills will fully dominate.
I still think the 2 suggestions I made are key to getting any semblance of balance in the late game. There are just too many skill points being handed out, allowing one to reach extremely high levels of any skill. Any effect ramped up to very high levels will be overly strong and too hard to plan for.
At the same time, the magic attack skills require sinking constant points into. In essence, the problem is that there are 2 different skill systems: one percentage based and requiring only few levels, and one direct damage based, requiring a ton of levels just to keep up. Putting those 2 systems on the same plane would make it possible to plan some kind of damage curve. Attack items give a decent damage curve with their stat requirements, and the fact that the player will gradually find better items means the damage curve is smoother than anything you can do with skill-based damage.
like the lack of level requirements on high-level mail/plate gear and weapons, so you can acquire level 100 gear at as early as level 20
I guess this has to do with the fact that there's a shared stash, and that the stat requirements are linear, while as you get good equipment, your stats grow much faster than linearly due to many items contributing to stats, making the higher level requirements meaningless. It needs to be a quadratic stat requirement function instead, or indeed just have level requirements.
In contrast, 90 to 100 in Din's Legacy can effectively triple your damage, move speed, defenses and clear speed simply due to the multiplicative nature of stats.
I consider this a problem. It may be a fun power fantasy, but the game clearly loses most of its challenge, when it's supposed to be at its most challenging at this point. I'm open to more suggestions regarding overpowered aspects and ways to correct them.