There are five main issues for casters.
1) Balancing a casters is difficult when all the different caster-classes share the same spell mechanics. There needs to be a way to scale spells for one class without indirectly nerfing others.
2) Heroic DPS spells don't scale well within Heroics, and are pretty much useless in Epics. This doesn't even cover the number of spells that no one uses, as the spells were designed so poorly to begin with. Most every spell needs a pass to improve usefulness and ensure that heroic spells are most than just decorations in Epics.
3) Epic spells and heroic SLA's have ridiculously low max caster levels. Which is really stupid for a game where mobs have levels of stat-bloat that would make even the most casual player green with envy.
4) Reaper Penalties (RP) are in my opinion, the icing of the cake, or if you prefer the final nail in the Coffin. If the first three issues were not a problem, then the RP wouldn't of become the big deal that it is for DPS casters.
5) Gearing.... The last good expansion/ update for casters itemization was IoD. After that Gearing has become an endless slog to nowhere. Farming ultra-rares that really don't matter, not after the repeated stat crunches and very obvious attempts from Dev's to replace our gear set bonuses. With an augment based (sun/moon) system which is primarily designed to feed the DDO store.
Edit: I forgot to mention that our Dev's have a horrible habit of penalizing players for their resourcefulness and creativity. What is meant by this is that players always find a way to maximize a given situation to their advantage. Even if the intent of the game mechanics did not originally support it. Best example I can think of is magic missiles and Shiradi. Not only did the Epic Destiny get heavily restricted, but the MM spells used have been Nerfed to the point that no one uses them anymore. This exact scenario has played out dozens of times over the years, resulting in much of the games spells becoming largely cosmetic effects.