First thing I learned, and possibly the most important - I hate descending AC. With a gawd damn passion!
You know what? That surprised the shit outa me. I used to think that ascending AC was for wussies - who the hell can't figure out the basic math for combat using descending AC? Apparently this 45 year old Grognard has trouble figuring it out on the fly during game sessions. One of the best inovations of the OSR was bringing ascending AC to the classic rulesets.
Actually, it surprises me in retrospect that this wasn't thought of decades earlier - it does away for the need of combat charts and the like and greatly simplifies the one confusing quirk of "Old School" Combat.
I've learned that running with the true rules (AD&D 1e) and it's clone (OSRIC) is easy as pie. Sure, they don't line up exactly (experience point tables are probably the most obvious example) but all of that stuff is minor and easily handwaved. Besides, the OSRIC rulebook is so much simpler to access and reference at the gaming table than then originals - when running a game, the less time spent referencing rules, the better.
We never had "guest players" when I ran my ACKS sandbox, but I have had with our current dungeon crawl. There's a different mindset in a dungeon crawl that seems more acceptable to drop in / drop out gaming. Maybe it's that everyone has the same goal in a dungeon crawl - kill things and take their stuff.
I've learned that it's easy to house rule a system you grew up with, that the "cleave" rules from ACKS make a nice fit in an AD&D game and letting players multiclass is just damn fun.
Most importantly, you can, sometimes, successfully return to your first love decades after moving on to the supposedly "new and improved" AD&D 2e ;)