On
The Way Inn, we try to address this issue by individual caps. Every character is capped at levels 4, 6, 8, 10 etc and can only progress through the cap when a DM allows them to pass (maximum level on the server is 20).
The effect is that most people don't soar through levels by powerlevelling, and most people also play more than one character. This spreads the level ranges that are in play at any time through the whole range of level (looking at the current player list, I see every level between 2 and 10, five of them below level 5 which makes teaming up with a level 1 perfectly possible).
It's not the all out solution, but it works very well for us.