When a character casts a spell or uses an ability that doesn't target an enemy directly, that character doesn't automatically attack the nearest target after he or she is done casting/using the spell/ability.

Did you check your options menu under game to see if auto-attack was disabled?  If it is not disabled then it's a bug.  I generally have auto-attack disabled see I can micro-manage my spellcasters to target specific enemies; e.g. wizards and druids otherwise they go after the closet enemy or the one initially clicked on in group.  After they cast a spell I pause and give them orders.

The option is enabled, and it works in all cases except the one I mentioned earlier. The workaround is to shift-queue an attack command after casting a spell, which gets tedious after a while.