Platformers, preferably 2-D with great art.

Not really, back in the day I bought 10 or 20 games a year and now its to 2 or 3. The quality of good games is going down. I used the term MOST for good reasons; of course most of the games you listed are good (even though I could disagree on quite a few of them).

Thief is a good genre.
YES I KNOW WHAT A GENRE IS

There are games in almost all genres that I love, from Sensible World of Soccer via TrackMania and Monkey Island to Armageddon Empires. But if I were to write down my top-ten favorite games, chances are there would be quite a few CRPGs in my list. So I would probably have to say the CRPG (including hybrids with strong RPG elements, like Deus Ex) is my favorite genre.

MOST of the games you bought back in the day are good. And I could disagree on quite of few of what you may have purchased back then as well. In fact, except for a handful of titles (I would say as few as 4 or 5), most of the games in GOG's catalogue are piles of forgotten dried up excrement that most people just can't admit to accepting because they're as hyped up about the "old days" as much as kids these days are hyped about the next Halo.
You also simply demonstrate flawed logic where you are mistaking correlation for causality when talking about the quality of games today. Just because you happen to be buying less games these days doesn't necessarily mean it is caused specifically by a decline in the quality of modern games. There are many other factors that you outright ignore.
For one, the ever increasing complexity in developing videogames have slowed down the production cycles. So with fewer games being released yearly, naturally there will be a smaller proportion of games that are of acceptable quality for you to purchase. But less numbers is about quantity, not quality. If anything the increased attention to development expands the breadth and increases the depths of videogame experiences.
The cost that comes with bigger budgets and higher production values however forces developers to ask consumers to fork over more cash. The seemingly endless increase in price of videogames really limit your purchases of fresh off the shelf titles. Again, if we're talking purely about the quality of a game regardless of its value, videogames have only gotten better. Or that least, no worse than it was a decade or two ago.
But I'd still lean towards the side of improvement and innovation.
And lastly, I can only speak for myself, but within the last five years my budget for buying videogames have decreased dramatically due to other responsibilities as I do my best to be a "mature" adult. I can only assume many others here experience the same shift in priorities? I actually have an entire back log of modern excellent looking games that I have yet to get around to playing. Time, money, effort, and motivation are all factors that have had quite an impact on my game playing habits.
But to get back on topic, my favorite genre the FPS is indeed better than ever. You just have to be in complete denial if you tell me that Half-life 2: Episode 2 is a worse game than the original Half-life.

I haven't played the episodes yet, but I would rate the original half life over its sequel, is there much in the episodes thats worth paying for? Also, how did you find the episodic content, would you have preferred HL3 instead?
Personnally, I'm eager to see Black Mesa finished.

I used to love FPS's but I was always borderline on getting motion sick watching them. Now I get motion sick on pretty much all of them so I've given that up.
Even when I was playing them though, half an hour to two or three hours was usually more than enough. I can play an RTS for like 8 hours.
Favorites in order of preference:
1. RTS (not many I like though)
2. Turn-based strategy (I actually play these much more now and have for a long time)
3. Simple hack and slash action RPG (Diablo style, Titan Quest)
4. Racing (don't actually play these anymore, just super fond memories)
5. Casual (World of Goo, Mahjong, Tetris, Minesweeper)
Used to be a big fan of MMORPG's, but they suck too much time out of my life to bother with now. I learned my lesson after many years of Everquest.

More derailing... but Half-life 2 was just so beautiful and ground breaking for its time that you just can't help but be drawn into its believable world and indentifiable characters. The game was drawn out at times and the plot was a little thin, but Half-life's story telling has always been minimalistic.
Episode 1 was more of an extended tech demo of the new HDR and updated graphical enhancements that Lost Coast first showcased. More of the same, but slightly prettier and some minor plot advancement. It was just really a tepid segue into something more substantial which was Episode 2.
Now outside of it being an unbelievable value as part of the Orange Box, Episode 2 was amazingly breathtaking in its own right. Everything was stepped up from the physics, to the character models and animations, and the major story elements were finally explored. It took all the exciting set pieces and explosive action packed firefights that were in HL2 and condensed them down into a few short hours with minimal padding in between. Making the experience short and sweet.
As for episodic content, all I have to say is that getting an episode every year and a half or so beats waiting more than half a decade between sequels. As long as developers offer a decent chunk for players to bite into at the right price. Valve has done pretty well so far.

anything sandbox-y. The more open ended, the better.
where I can just do my own thing and not have to worry about goals or stats or money
and I'm a sucker for the RPGs, be they MMO or otherwise

Role Playing Games and strategy