The primacy of adventure over story.
The primacy of surrender to the wonderful, the fateful, and the weird over rules mastery.
And Appendix N. Not Gygax's specific list, nor any other, but the attitude behind it in which one seeks to take the best elements of the exciting and the evocative and bring them quite literally to the table.
If you are interested in this sort of thing, Tenkar's is a great site to visit. Not grandmother-friendly, perhaps, but a consistently fun and interesting read.
A lot of folks naturally think of old school D&D when they think of the OSR, but it encompasses more than that. Seventies & Eighties gaming is probably a better characterization of its focus. Most of what we play today is really just a remix and refinement of what we played in those earlier years. The OSR crowd, at its best, seeks to recover and revitalize the finer elements of those games.