What he said.
It's OD&D+ supps/AD&D through the lens of a modern (for the time) D20 system. The spirit of the game hearkens back to the minimal rolling, fast, loose,and minimum amount of rule necessary. It was OSR/Retro before the OSR.
It was also designed to be on the fly compatible with the TSR editions of the game, in particular all the adventure products available since D&D's inception. It is also very easy to convert much of the 3.0 material from WOTC proper and the D20 market (3.5 less so, due to it's added reams upon reams of changes large and small). Stat blocks for creatures are similar to old B/X D&D products.
It is lighter than any WOTC version of the game- and it's lighter than AD&D/OD&D + supplements because the systems are tighter and easier to teach (everything runs off simple ability checks). It is also highly hackable/moldable- I've added all kinds of things from other games like 13th Ages Backgrounds, and Monster "Specials" and they plug right in. I've beefed up the Fighter class with some 4E-isms.
C&C gives a bit more AD&D options with it's class/race choices and spells, but it plays more like B/X.
The Trolls also put out a bunch of good adventure material.Some people are put off the by the attempts of the Trolls to write the books in "High Gygaxian" (and frankly the Trolls just don't quite cut the mustard at it) , so the books could probably be a lot less page count than they are. That said, I have come to enjoy their style/attempt vs. reading so many rulebooks that come off as a school textbook and either put me to sleep, or turn me off a game.