Back when 1st edition was the newest edition, which was when I started playing, we used the crap out of Appendices A to E. A, B, and C probably saw the most use. These were the ones with random dungeon terrain, random wilderness terrain and random monster encounters. They were immediately useful in the preparation for and application during actual play. You used the fuck out of these; in fact, outside of the sections on magic items and gems, those three appendices were undoubtedly the MOST used sections of the entire book for me. It was a big part of where I got my love of random tables, and also learned the lessons about using them properly (for example, making a dungeon with Appendix A 'by the book' would almost never work right, so you needed to learn how to adjust tables to fit what you actually wanted and what actually worked).
Appendix D was for "lower planes creatures" and became a huge inspiration for weird and crazy monsters.
Appendix E was purely a practical section: it listed in pure stat-block all the monsters, which seems not that sensible but was in fact essential for a young kid that didn't own the Monster Manual yet, or for someone who didn't want to carry both books around with him. In the days when a D&D game might happen anywhere, and where you already had a backpack full textbooks, that was damn useful!
Even beyond these key choices, though, ALL the other Appendices had some basis in actual PLAYABILITY. Be it tricks, traps, summoned monsters, or the gambling rules; they were all for using.
All except Appendix N. In actual history, as I lived it at least, Appendix N was the one you just skipped over. The only memory I have of it was once or twice comparing with friends as to how many of the books on it we'd read, and it was always a near-tie, because we'd mostly all read all of the books that were actually popular and none of those that weren't.
Now, even if you were to believe the nostalgia and ideologically-driven delusions of certain OSR segments, even then Appendix N wasn't for using; it was for ruminating on, and thinking deep literary thoughts about, and assigning a seriousness to D&D that in no way matched how we tended to play. But no one I knew did that kind of bullshit back then; not until "Vampire: the Masquerade" showed up.
Which kind of makes sense, because the retroactive "importance" of Appendix N was largely invented by a reject White-Wolf fanboy and total johnny-come-lately to Old School D&D: James Maliszewski.
Appendix N's popularity only arose because of this entryist, "JMal": an internet kickstarter fraud, World-of-Darkness fanatic and pretend OSR guy, who only got into it when he had the sense to see that White Wolf was dead and that there was rubes to fleece and money to make off the OSR. It makes sense Maliszewski would claim to love and promote the endless study of the minutiae of Appendix N: it has no play content, but tons of pretentiousness-potential. Appendix N itself was nothing more than just a list of 'cool shit Gygax liked', but in the hands of Maliszewski and his cohorts it was all about pretending to be literary critics and getting to be judgmental about what is "real old school", and finding some kind of quasi-esoteric "primordial UR-D&D" to show you're more old-school than anyone else.
It's all about trying to push an OSR that's exclusionary and reactionary, rather than innovative and creative.
If you want to do stuff that's about creativity, look at EVERY OTHER Appendix in the DMG. Let those inspire you. Let the random tables and the lists and the ideas for play inspire you, rather than looking for some kind of bible of Gygax-Approved books to tell you the only right way to play D&D.
Nobody is suggesting that you not read the books on the list! I've read quite a lot of the books and authors there myself, though certainly not all.
What I am saying is that the J. Maliszewski Serial Wankers Club For Talmudic Studies that has formed around the least-useful appendix in the DMG has chosen to dedicate hours to the study of that Appendix N, and not to Appendices A, B, C, D-M, or O or P, because N suits a goal of creating the attitude that the way one group thinks old-school should be run is the 'right', 'true', 'original' version of some kind of primordial Ur-D&D of which all other versions are just sad falls from some golden age that never was.
If you think the OSR should be about innovation and creativity, about how to create, within the 'box' of the design rules of old-school, amazing NEW stuff, instead of rooting through the Gygax family home's garbage bags in search of old shopping lists to try to get some grasp of how to play D&D as 'purely' as possible, then I would strongly recommend you try to put some real hard time into carefully examining, studying and experimenting with every appendix in the DMG except for Appendix-fucking-N.
Currently Smoking: Italian Redbark Billiard + Argento Latakia