Sirlin has written what seems to be the most commonly referenced piece whenever games and "thinking" come up. Playing to Win, available free at his website (www.sirlin.net/ptw/), doesn't directly deal with game design, but it comes up throughout. His blog generally trends on game design topics. (He's been a fighting game pro, rebalanced SF2 for the most recent version, and is in the process of releasing some physical games now.)

Costikyan writes a lot of good stuff. http://playthisthing.com/randomness-blight-or-bane is a great example, and you can find some other writings of his in various archives. PTT itself has him (and a few other contributors of varying flavors) writing on new (indie, board, etc.) games.

Soren Johnson (Lead designer on Civ4) had a series going at http://www.designer-notes.com/, but hasn't appeared to have posted recently. http://www.designer-notes.com/?p=132 is a good example, discussing how easily players can perceive an AI to be "cheating".

Brenda Braithwaite (Wizardry series, Train) has a similar, seemingly modestly defunct blog that is worth reading regardless.

Edmund McMillen of Super Meatboy posted about difficulty: http://supermeatboy.com/13/Why_am_I_so____hard_/. There's a similar post somewhere on the blog.

Gama Sutra occasionally has useful things, but you need to filter a lot.

Without endorsing it overly, Less Talk More Rock (http://boingboing.net/features/morerock.html) is worth consideration. (For balance, http://sinisterdesign.net/?p=457).