One simple hack I've considered is to only allow PCs to level up once/month. Or even rarer, like once/year. If they have enough XP to level more than once they have to wait an additional month+ for each level.

The DM gets to plan adventures with a fixed power level in mind. Players can use smart tactics ("let's go straight for the bad guy instead of killing every creature in his stronghold first") and not worry too much about a TPK. PCs level up only after they've had time to train, meditate, research spells and so on. Players have a reason to consider their characters' lives beyond dungeoncrawling.