What Old School fans agree on is that older editions are much fun to play today as they were back in the day.
There is nothing in the Open Game License that requires me to use the entirety of the d20 SRD. If I don't use feats, multi-classing, sorcerers, very long stat blocks, limit the monster abilities, etc; what is left is a game similar to the older editions. This is the fundamental foundation that underlies much of effort that goes into publishing for older editions.
I know these comments may seem like Old School 101 but after reading the Nth long winded discussion about the OSR I felt it would be good to restate the basics.
As for why people are writing and publishing Old School material I refer you to the quote in the upper right of this blog.
Then there is the Old School Primer which is helpful in learning how to make the minimal rules found in the oldest editions your own.
Finally the most important basic observation .. Have fun!