Turn-based combat works best if it has a good amount of strategic depth. Like chess, the turn-based nature helps you plan ahead and makes it more a thinking game than a reflex game. However, if the combat doesn't offer any depth then it becomes unnecessarily slow and tedious.
It's the main reason I love turn-based combat in some games (X-COM, Jagged Alliance, Temple of Elemental Evil, Civilization, Alpha Centauri, Final Fantasy Tactics, Blood Bowl, Final Liberation etc), but can't stand it in other games (most jRPGs).
The average jRPG amounts to "a couple of guys standing in front of each other taking turns beating each other up" and contains almost no strategy beyond "use fire magic on the ice monster". That's all well and good in a game like Diablo, but it's frustratingly boring in a turn-based setting.
Personally, I prefer good turn-based combat over any kind of real-time combat, but I'd take mediocre real-time combat over mediocre turn-based combat any day of the week.