Most (all?) direct2drive games have modified/protected .exe files which is hell if you are into getting mods/fan patches for games. While steam has similar methods of ensuring file integrity they are less intrusive and most stuff still works on steam versions.
Conclusion:
As european, farm (uk) direct2drive sites for "meh, not really worth it games" for under 10 pounds if you want a bit of singleplayer entertainment.
If you want full priced, new games with a big multiplayer aspect check on Steam.
If you want good, full priced (possibly mod-able) singleplayer-focussed games buy/order boxed versions.
For good old games, gog.com
If you are desperate you can check out Impulse, its a bit less intrusive than steam but I have yet to see a single good bargain on there.