embarassingly i have quite a few games that i bought on GOG which i never beat until recently this year when i had more time to play games here's is the list of games that i never beat during the first playthrough

The 7th guest

when i first played the 7th guest, i got to the end of it and just started to get bored so i decided to just stop playing it but that was completely wrong of me

Overseer

I got to the last part of the game where you were on Alcatraz but i could never time getting out of the cell right and i'd always be spotted by that damn security robot dammit

Gabriel Knight 2 The Beast Within

Another great fmv game that i left unfinished for a bit. i got to the finale where gabe had become a werewolf however i had a lot of F#$@ing trouble closing the damn doors, it pissed off to no end and i raged quit only for a few days this time

Police Quest 4

Okay, okay this will be the last sierra game on the list i promise. See i never could figure out what to do after a certain point in the game, i feel like i might have encountered a glitch. I stopped playing it and didn't complete it. However i did manage to beat a whole 1 year later, i'm dead serious

so there you have my list of games that i left unfinished however i have finished them now

*Looks at backlog*

Er... where do I start? Suffice to say I don't think that four games is all that bad.

I've got about 125 releases. I believe I've finished 9 of them.

My list of gog games that I own and have yet to beat is quite manageable:
Sanitarium: Not even started this game yet, though I've heard so much good about it that I really should.
Little Big Adventure: Started it, but thought that the controls were really awkward. I'm still interested in finishing it though, I'm just waiting for a slow day, where I can really sit down and focus on the game.
In Cold Blood: Bought it on a whim, it looks interesting, but after giving it a try, it felt like one of those games that I really need to be in the right mood to play. It seem fun though.
Shattered steel: Not played this yet.
Arx Fatalis: A CRPG must play, I'm just waiting for my next CRPG itchs, and I'll be all over this game! I have tried it in the past, and it looked very promising, I was just not in the right mood to play it then.
Empire earth Gold: I doubt I'll ever finish this game. I did not like it :(
Total Annihilation: I played it for quite a while, but it got a bit repetitive. I'll probably return to it at some point.
Realms of Arkania 1-2: Same as for Arx Fatalis. I've played RoA 3, and I loved it, so for these games, it is just a matter of time before I'll plow through them.
Lure of the Temptress: While I love adventure games, this one was not my cup of tea.
Dragonsphere: Not played.
Teen Agent: Not played.

As for my actual backlog:
http://www.backloggery.com/fnord

Most, if not all, RPGs I've played. The notable exceptions are the Gold Box games. From my GOG era:

Stuck on the very last level of Far Cry and I want to get through it without cheats.

As mentioned in another thread recently, stuck early on in the campaigns of both Soldiers and Panzer General.

I haven't beaten X2 (barely scratched the surface, actually), but the more I read of the game online, the more I'm thinking that you don't actually beat it.

Life, though I'm making a good attempt at it.

There is only one game I have never beaten in my entire life.

Empire Earth.

Only because I can't get it to run. I'm a perfectionist when it comes to video games. I even played through the old E.T. game on the Atari just so I didn't ruin my streak of never giving up on a game.

I have beaten a grand total of 2 GOGs.

Lands of Lore 2 – I could never stomach the ugly graphics engine.

Betrayal at Antara – because of numerous bugs and crashes. There's a patch made by GOG staff that fixes some of the problems, but I moved on to other games in the meantime. I may return to it during a dry gaming period.

Alpha Centauri – purchased during a promo weekend, but I haven't been in a turn-based strategy mood for quite a while.

Gabriel Knight 2 and 3 – I don't like the second one's FMV graphics, and I refuse to play the third one until I've finished the second one.

The four free games I got along with Witcher 2 – if they were any good, I wouldn't have gotten them for free, would I? Then again, I may be wrong.

EDIT: If we bought only those games that we mean to actually play through to the end, GOG.COM would be a lot poorer. ;)

I think that's a far more realistic way of looking at it for most GOGlodytes.

From the GOGs that I own, I've completed:
Gothic I, Gothic II
Tex Murphy UAKM, PD, Overseer
Duke Nukem 3D
Fallout 1, 2, Tactics
Broken Sword 1, 2
Unreal Tournament
Arcanum

So that's 13 games, but most of them I have completed more times than I can count. For example, I have played through Fallout 2 at least 50 times. Not even exaggerating.
From that list, the only games that I've completed only once are Broken Sword 2 and Arcanum.

I have also got really far in some games without finishing them though.

The list of games that I HAVE beaten would be a lot shorter.

I guess the most notable game around here that I haven't beaten would be The Witcher.

One of the things that I have started doing lately is limiting the number of games I have installed in the hope that I might actually start to finish a couple.

The game actually has an end, though the end is not really the goal or the point of the game. Oh, believe me, you are! Well, as far as Beneath a steel sky & Tyrian 2000 goes at least. Ultima 4 requires a lot of dedication, and is more of a historical curiosity than a great game. I'm not saying that it is bad, mind you, just that it might not have aged all that well. I tried to get into Lure of the temptress, but that game did not really appeal to me, and I've not tried Teen Agent or Dragonsphere. (By the way, that is 6 games, not 4 :P )

That is, of course, if you are talking about the free GOG games, and not some other free games specifically related to The Witcher 2.

When it comes to RPGs, I suffer from Chronic Restart Syndrome.

This year I'm hoping to finish at least one of Gothic II, Throne of Bhaal and The Witcher.

One of the first games I got from Gog.com..Dungeon keeper,I was very near the final level but then my graphics card stopped working,I have since re installed them but now I have to start from level 1.

Also Thief Gold.

I've got a total of four free games on my virtual shelves – the ones you mentioned, but without Ultima and Dragonsphere. I've heard good things about Beneath a Steel Sky, but of all graphics engines Sierra used in the past for its point & click adventure games, this one is, IMO, the worst and the buggiest. It's also used by KQ5, PQ3, and the new Gemini Rue.

I've got about 175 games on GOG. I've finished...Redneck Rampage.

174 to go.
Huh?