This is what you get when you build a game without any discrete power levels.
Most (99%) games have real and fairly tight thresholds on what is too tough to
fight and what's too crappy to bother with except for a laugh. There are a lot
of reasons for this, and one that no one with money at stake has ignored. I
suggest making high-level players a LOT stronger, making it harder to get to
high levels(though means besides xp costs), and making low level monsters less
valuable to them. Sorry for posting this, I'm quite tired and strung out on
caffeine and my judgment is so shot I think it's important to say this for
some reason.