not to mention it gives casters a power no one else has...
Other: They just don't make sense within the context of D&D. We have this nifty game resource to protect PCs from all the horrible deadly things they're expected to survive. (Hit points.) And then we go and write spells and traps that arbitrarily bypass this protective resource. Doesn't make sense.
Firstly, there's the lethality aspect. They make it very easy to kill a character, which then either means hours sitting out of the game (creating a new character and having him written in, or a lengthy quest for resurrection), or they require that the game has easy resurrection, which I detest.
Finally, on the caster's side, they tend to be very much all-or-nothing. The evil Wizard enters combat, cackles evilly, launches his most devastating death spell... and all the PCs make their saves, and come through with no effect. And in the worst case, that may prove to be the only attack that the Wizard gets to make before he gets minced.
So I think I'm leaning towards a model where SoDs become "if you hit you get a lesser effect; on a critical it becomes SoD" as my preference. That way, they're much less immediately lethal (since a true SoD becomes much more rare, while still being a real risk), they require two rolls to kill a character (or, even better, three if criticals require confirmation), and they cease to be all-or-nothing effects.
(Plus, that also has a nice symmetry with my notion that a Fighter, on a critical hit, could choose to forgo the extra damage and instead disarm/sunder.)
Other: They just don't make sense within the context of D&D. We have this nifty game resource to protect PCs from all the horrible deadly things they're expected to survive. (Hit points.) And then we go and write spells and traps that arbitrarily bypass this protective resource. Doesn't make sense.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.