What the games do have in common, however, is the "grognard" subculture of bloggers/gamers and the game designers that serve them, and the rapid communication between them available because of the Internet. This cause and effect relation, found only in the 4E release->OSR rather than the 3E release->OSR, implies that while the game system assumptions that define the "old school" are *ahem* old and pre-existent, they are not the cause of the change.
Unless one wishes to tie Piazo's Pathfinder into to the "Old School Revival" as well and in so doing defy categorization of the meaning "Old School" other than pre-extant gaming, at which point we're calling the OSR a beast fed on nostalgia and bitterness.
And we have a winner.
First off, how "Old" does an OS game have to be? 1970's is a given. What about games made int the 80's? Or the 90's? (if not, you exclude BECMI and 2e). Is the only qualifier being OOP? (Which technically allows 3.5 to be OSR).
Secondly, how "lite" is rules lite? Are we talking core rules only or supplements too? (AD&D and OD&D get remarkably complex when you factor in supplements).
Third, how do we define lite? 4e is lighter than 3e, not as light as OD&D, but is it lighter than 2e? Than 2e + Skills & Powers?
Fourth, what about retro-inspired (Hackmaster, C&C, BFRPG) and retro-clones (OSRIC, LL)? The strictly break the age requirement, so what do we judge them on? Closeness to original material? Liteness? (if so, does True d20 count?) Something else? (see below).
Lastly, What qualities (other than rule density and age) can lump these games together? Are we looking for specific rules (thac0, save vs spell), or tone (the 1e tone is vastly different than 2e's tone, but striking similar to 3e's)? Is it authorship (Only games penned by Gygax himself)?
What gets included in the "OSR" cannon? Until these traits are defined, it IS just a feeling, and that feeling will differ from poster to poster, board to board, DM to DM. The individual player/DM might have reasons beyond "nostalgia" to play (easy of play/prep, tone, etc) but the movement as a whole is nostalgic unless you can define what an OS game IS.