The Great Reset of 2006
On February 5th, 2006 the entire player database was reset. All accounts that hadn't been used with a month of that date were deleted, and all other accounts were set back to first level. The reason for the reset was to implement a new experience point system that kept players from reaching levels that were too high to effectively challenge with the existing monsters.
The Great Reset of 2010
In September 2010 the player database was reset and several aspects of the game were reworked. The new HP system replaced one where HP increases occurred only on level-up. With many vitality raising boosts this allowed players to reach very high HP and caused the creation of Soul Imps which would lower the max HP of any player over a certain (class-specific) threshold. Soul Imps were removed with the new vitality system making them obsolete. Clans were removed and new trainers were placed around Ardania. The East Cheswick Trading Post, which sold a wide variety of items (and made high-level items available to low-level characters), was removed. Several monsters were altered or created and several locations were changed (notably the Dark Forest, which became considerably more challenging).
You can read in the forum http://www.heroesofardania.net/mbbs/forums/thread-view.asp?tid=5152&posts=12&start=1#M79908 about all changes.