You gain levels by accumulating more and more experience. The higher you climb in the world of levels, the more you unlock; like more gameplay features.
Level 3 - Ideabox with basic sections.
Level 5 - Godpower Accumulator.
Level 7 - A motto for your hero.
Level 10 - Duels and Ideabox with a few more sections.
Level 12 - Guilds and ability to vote for chronicles.
Level 15 - Skills; ability to edit the GodWiki and vote in the Ideabox.
Level 18 - Ability to rear pets.
Level 20 - Ideabox with all sections.
Level 39-60 - Temple; once constructed (depending on the rate of brick gain).
Level up once; an increase in maximum hero health of 4 and free extra charge for the Godpower Accumulator
Level up twice; an extra slot in the hero's inventory to carry more items.
Level up thrice; an additional skill (after level 15).
As an added bonus; when a hero levels up, her/his health is fully restored and the Godpower is refilled to 100%. He/she also gains access to higher level weapons and armor. As a hero's level climbs, so does his ranking in the pantheon of might.
The higher your level, the more experience you need to reach the next one. This is why growth is fast at first, and then becomes frustratingly slow.