Old school gaming gets defined differently depending on who you ask.
The two most common definitions—that an old game is "old school", or that the earliest edition of a ruleset is "old school"—aren't what do it for me. I don't think that Everway is old school, nor do I think that 1st edition Vampire: The Masquerade is. The definition of "old school" that I subscribe to is a style (or "school") of play that developed in the early years of the roleplaying game hobby. Although both old and modern styles are "roleplaying", the basic assumptions of how the players, the referee, characters, and world interrelate are completely different. Not better or worse, but different, and the superficial similarities make it hard for players coming from different backgrounds to appreciate the enjoyment that comes from each.
I wanted to sum up the differences succinctly right here but ended up deleting every attempt. I'll just give a link to the PDF "A Quick Primer for Old School Gaming" by Matthew Finch, which was the point of this post anyway. As it says in the blurb under the download link, "open-ended rules ... are USED very differently than rules are used in modern systems". I think anyone who is going to be giving pre-1990s rules (or new rules designed to support old school play) will increase their enjoyment tenfold by reading this brief primer. It's also well-written and a pleasure to read.