In a slightly different approach from the others in the thread, I ban all spells that give a bonus on
attacks,
saves,
AC, and/or
checks of any sort, and all spells that grant
temporary hit points, and spells that grant
damage reduction, and the
negative (
debuff)
versions of the same, unless that effect (whether bonus/penalty or temp HP or DR) is paired with some other, more interesting/unique effect. In other words, boring spells are not allowed.
The reasoning behind this is that magic in one of the best things about D&D 3.x, and to use it for something less than exciting is just such a
waste. Fiddling with the numbers wastes time, and banning them prevents the instinct to use those spells just because they are effective rather than because they are actually fun to play with. The group does not miss them, and the use of magic stays exciting.
We also ban a few other spells, like Knock and Divine Power and Celerity and similar spells. But the bulk of banned spells are banned only because the are not interesting enough.