Dota items are awarded periodically for a number of different activities. The longer you play and watch Dota, the more drops that your account will receive.
If you feel that you have encountered an issue with Dota's item system, please report the issue via the Dota Developer Forums.
Timed drops are awarded to players simply for completing matchmaking games; win or lose. The type of items you can receive are full item sets, bundles, treasure or Arcanas.
Accounts that are spectating live tournaments have a small random chance of obtaining an item when a notable action is performed by the game's players. These actions include:
Steam and Dota accounts that are currently in any of the following states do not qualify for item drops or achievements:
In some rare cases, a game may be discarded prior to rewards being granted. Unfortunately, in cases such as this, there is no way to restore these rewards. The game and achievements will need to be completed again.