Price Glitches - Chances are you can get a brand new game for 7 cents because computers are hard. Then when you whine about it enough, you'll probably get added bonuses on top of it for being such a gem of a customer and human being. Herp derp!
Gifting - Buy a bundle that has a game you already have? Most DD services you'd have to just count it a loss, but with GamersGate you can actually gift individual games to other GG accounts as long as you haven't downloaded them or revealed the serial code.
Generous
Refund Policy - If you don't use the game you can get a refund within 14 days, no questions asked. Beyond that you can still get a refund if any technical problems persist and a solution can't be reached with support.
Blue Coins - Earn coins from each purchase, rating, review, and more. 1,000 blue coins is worth $1. You'll basically rack up credit for new games just by using the site.
Game Availability - Bundles tend to contain more games as compared to their Steam equivalents, and European games that aren't normally available to Americans can be purchased.