* No DRM that prevents me from installing or playing (BGaE retail won't install because of it, my old retail copies of Quake 3 and Half-Life claims that the CD-key on the jewel case/manual is invalid - WHAT?)
* No discs that get scratchy or lost (had to rebuy Age of Empires 2 because I lost the disc, although as a bonus I got Age1, as well as their respective expansions)
* Good games that I can't find in brick&mortar stores any more (please try to get KotOR to here soon... 'kay?)
* Easy (I feel like playing something new so i fire up the web browser and go here, see some interesting game and click buy - done, I can download and play)

Buy MOAR NAO!! *whips Maggie with Death to death*
<3

It definitely feels much much better to make a $10 mistake than a $50 one.
To me, the $50 one's better graphics don't make the risk more worthwhile. I know a lot of people don't feel that way at all, but to me the gameplay is the thing and new games have little if any edge in that regard.

Nostalgia
Price
Lack of DRM
XP compatibility
Extra's like soundtracks
Community

I'm a cheap bastard, I like cheap games. But I don't like piracy.
I also tend to like a lot of older games, as so many of the modern ones they're mucking up. I mean how many "PC" qames have been build for the XBoX and ported to PC? If you're lucky, it's a decent port, but so often you're missing our horribly if you're not playing with an XBox controller.
I also like the concept of GOG. And the no-DRM policy. As DRMs suck.

Prices, people, and lack of DRM.
GOG has a great price point, great community, and LACK OF DRM!
Seriously, it can't get better than this.

Low prices, guaranteed compatibility and additional material.
And of course, I love old games.

There are several reasons. But mainly because they sell a number of interesting games that I can't (as easily) buy anywhere else.

I come here because I consider CDProjekt to be the best company in the game industry today and I love them.The fact that the service is awesome and games great (not just good) also helps, as does the great community of users around here.

Quite simple
1)Low price
2)Lack of DRM
3)Games are good
4)I love old games
5)Good Service
6)Guaranteed to work so I don't have to deal with idiot support ever
7)If for some reason I need to contact support here it is reliable and the people know what they are doing
A little off topic but nice avatar ;).

Availability, convenience and compatibility.

I tend to be pretty pragmatic in my purchases, so my reasons for buying games from GOG are quite simple: they offer quality products I'm interested in (good games, very few headaches (compatibility, DRM, etc), and very rapid and simple purchase process) at very reasonable prices.

Cause I'm cool and retro software is so in.
Or maybe it's because these games are just as awesome as when I first played them (Screamer) or I didn't have a computer that could run it when it first came out (I'm looking at you Mesiah). Either or.

im too young to have played a lot of these games, but i've read about them and heard about how they've influenced gaming and how they are classics, and I feel good about playing them and knowing exactly what they are like, as opposed to just acknowledging that they are classics without trying them. it makes me feel more like i can call myself a real gamer.
plus, graphics get worse, but gameplay and fun factor stay the same for the most part, so many of these are just as fun as games that cost (literally) ten times as much. it's the sacrifice i make in these tough economic times :p

1) No DRM or regional restrictions
2) no third party software shoveled down my throat (Games for Windows Live, Steam, Impulse etc.)
3) Ensured compatibility & latest patches and expansions included
4) Price
5) Community, support and service in general
6) I like gameplay and content more than fancy graphics (and I don't have to replace video card or upgrade my computer otherwise every year to be able to play games with it)
7) by supporting the service there's change I see other games I'm interested in show up here eventually