I would think that going from one level to the next would not be impossible, but rather difficult. Having each level require a double amount of points/experience and what not to move to the next level.
Example:
Level 1 0 - 2500
Level 2 2501 - 5000
Level 3 5001 - 20,000
Level 4 20,001 - 500,000
and so on. The limit? None.
Reaching maximum level should not be easily acquired, in my opinion, there should never be a maximum level or highest experience to be attained. If there is a max level or experience to reach, then the game is limited with what it can achieve.