Well, this is an interesting issue, and I tend to see both sides of the argument. As Warp (for example) mentions, games take real resources (time, money) to produce, and so those who expend those resources deserve to be compensated for their investment. On the other hand (as many have stated), games are expensive and I do not have an excess of money to spend on them. I like to think that my enjoyment of a game is at least a bit of repayment to those who have developed it, somewhat like the satisfaction I and others get by investing time to make TAS movies. Of course it doesn't take money to do that etc but I think the point stands.
That said, let's look at the recent games I have purchased (or at least those I remember purchasing):
World of Goo ($20) -- I have always enjoyed bridge building games, and there was a lot of positive press on this being both very well done and also being independent and not protected. I played through it once and my partner also enjoyed a couple of levels, which is of note as she is not much one for video games.
Braid ($15) -- I decided to buy this for PC when it came out after playing the demo on a friend's Xbox (and perhaps also because like World of Goo it had positive press relating to gameplay and being independent). I still have not beaten it, due to a combination of not being smart and also it not running terribly well on my sort of old PC (I do have vague plans of beating it someday; I have done about 75% of the levels and enjoyed them).
Audiosurf ($10) -- I got this after it was mentioned on these very forums. It was fun and the price point was right. I don't play it any more, but maybe I will someday. But, I have probably played it at least as much as the games above.
Portal ($5) -- Got this on sale on Steam at the same time as Audiosurf. Great price! Unfortunately it does not run well on my computer and also I am not good at first person shooters (even if this isn't much of one, it still is). So, I have not played it except for maybe an hour or less when I first got it. Again, good press on this (and word of mouth from friends too actually), though this time mostly on it being a very cool game.
The Fool and His Money ($50) -- I paid $50 for this and I still don't have it. However -- I am very excited to be getting it when it is done, hopefully this year. It is a puzzle game by Cliff Johnson who made two of my favorite games, 3 in Three and Fool's Errand. They all feature interesting puzzles plus a lot of "meta puzzles" where clues from all over the game help. These earlier games are available for free (!) at
thefoolandhismoney.com (The appropriateness of the title of this game in juxtaposition with my current situation vis a vis payment and game-having has not escaped me.)
So I guess what makes me buy a game/not buy a game is knowing about a game, the game's politics, and the price point, all weighed by my irrational mind.