Here's something I've been mulling over for a while: should magic-users be able to dispel cleric spells?
I've been thinking for a while that they shouldn't. Block them, maybe, but dispel them? No.
For one, if you're sticking to the themes of the original spell list, most cleric spells aren't persistent unnatural effects; they're immediate, natural changes. There are a few exceptions, but they are mostly support/protective effects, like Protection from Evil, Speak With Animals, or Create Food and Water. So most cleric spells don't leave some enchantment floating around that could be dispelled.
For another, we're either talking about divine intervention (standard interpretation) or the work of spirits commanded by a saint (interpretation I've been using.) Either way, that doesn't seem like something a magic-user can undo. Another cleric could, though, with Dispel Evil.
There's also the difference in wording between Dispel Magic and Dispel Evil. The former is "effective in dispelling enchantments of most kinds" (emphasis added.) The latter dispels "any evil sending or spell" (again, emphasis added.) Maybe Dispel Evil should work on every spell, but there does seem to be a superiority over ordinary Dispel Magic. Before the craziness of AD&D, Dispel Evil was also your go-to banishment or exorcism spell.