When Valve introduced the first four class updates to Team Fortress 2, an achievement system was used to help unlock new weapons and items. According to Valve, that system did not work as intended.In the end, instead of playing the game toward fun and rewarding goals, hardcore players would grind achievements to unlock the game's latest weaponry. At the time, users complained the achievement system was changing the landscape of Team Fortress 2's online community - mostly because some of the achievements were so difficult to acquire regularly.
In response, Valve has changed the new weapon system with the latest update (Spy & Sniper). In the new system, Team Fortress 2 utilizes Valve's Steam Cloud, automatically rewarding players new weaponry and items previously only attainable through completing achievement related tasks.
Full post on Joystiq here...