1-5 are some times to rough for first time players, the fact that death is easy to catch is hard to grasp.
6-10, in a world where maybe, of a like ours, a 3bill popoulation, maybe 1mill exisit at or near this level, your pretty big and powerful.
We always counted your level into the population by adding a "0" to it so:
1- is 1'n 10
2-1'n 100
3-1'n 1,000
4-1'n 10,000
5-1'n 100,000
6-1'n 1,000,000
7-1'n 10,000,000
8-1'n 100,000,000
9-1'n 1,000,000,000
(From here on, powerful beings either leave the planet into other realites, or fall away from knowledge, or live amongst people hiding themselves.) Power corrupts, and ultimate power corrupts absolutely...many, as I see it, kill each other for more power, because, well, nothing in this system would give them more experience....they could kill half the population and not much would happen as rewards for them. Also, you got to put in ageing, natural ageing allows for most heroes to die before they reach levels beyond 7 I would say, not including the players, sinc the story is set around them. And since the game is about evil and good, good is always more powerful, but alone...so evil beings, or those who had done evil once, were usually the most powerful. (Not including other reasons, like divine influence, curses, destiny...bla.)
I like 1-10 best myself, lots of things are still dangerous.
11-15, its fun, you got legends going on around you about you.
16-20, well, the story's got to end around this time, because what can stop you.
21+, I as a Dm who's ran one game from 1st to 28, was the highest player, started wondering what was the point, armys were nothing, legions of men could be killed with any magci users will, epic fighters and barbarians mowed them down, rouges an monks never got it...even dragons at this point weren't really that scary to them, so whe nteh game ended, I said no more epic level games.