Is there any penalty for resting? It kind of feel like it makes the healing potions redundant.
There are two penalties for resting, actually:
1) You're given a "score card" at the end of the game, which shows you a ton of stats and gives you a title based on them. One of the stats it keeps track of is the number of times you rest, so if you're shooting for the best title, you'll want to keep your resting to a minimum. (That's why you see little tick marks on the screen as you continue resting; to prove your shame to everyone looking over your shoulder!)
2) When you rest, time moves around you in fast-forward, and your defense drops to like, zero. Even a slime can hit you for 255 damage and kill you instantly if it happens to catch you mid-rest, and if you rest for more than a split second or two in a room where ANY enemies are known to spawn, it's pretty much a death sentence. Effective resting can only really be done in closed rooms with no enemy spawn points -- so if you really need to rest, be sure to close all the doors before doing so, or you might be in for a rude awakening!
There were actually moments when I desperately needed to rest, but couldn't really go anywhere that I knew was safe, so I would literally press the SELECT button once to rest, then press it again a split second later to wake up, make sure there were no enemies around, then just repeat over and over again till I was healed. Which might seem like paranoia, except that on more than one occasion, I "woke up" to find a monster like, right next to me, swinging its weapon -- meaning if I hadn't been taking such short "naps," I would've died for sure.
It's kind of a neat touch, I think.
-Tom