Interesting topic - to which there's no 'correct" answer unless the organization you fly for dictates the gear to be worn.
The AF directs cotton underwear, nomex/aramid thermal underwear (in winter), nomex/aramid flightsuit, nomex/aramid AF-approved flight gloves. AF approved-for-flight boots include the Addison FWU-8/P black leather speed-lace type, and Belleville models 690, 700, 770, & 790.
Although we can wear Goretex gear in foul weather, it cannot be worn in flight. When the A-2 leather jackets came back, there was a period when they were not to be worn for flight, but that was rescinded. Many of us had the A-2 lining replaced and this red-lined those particular jackets. Of course we have summer and winter weight nomex/aramid jackets for normal ops and the Deep Freeze guys have another wardrobe to choose from.
Herk crews typically wear headsets, although we are to carry the HGU-55/P (or similar) helmet on any mission when chutes are carried and bailout is a possibility, ie. combat, tactical training, etc. We also wear the helmets for night ops (as the NVG mounts attach) or for flights in a combat environment. Helmets certainly provide more protection than a headset, but I think their protective powers are overrated for those who have never held or worn one (speaking of the current AF issue type). I suspect a number of crew have bailed out and reached the ground missing visor, helmet, or both.
Our Life Support trainers way back when even cautioned us against using lip balm (or, for females, other petroleum based makeups, etc) for fear of assisting an oxygen-fed fire. Finger rings and neck scarves are a no-no.
That said, in the course of a career, you see alot. Rent-a-cop boots, sports undershirts, goretex jackets, whatever. Sometimes the person is corrected, othertimes, not.
For my AF flying, I've clipped the tip from my right glove index finger and thumb. That's a no-no, but I found I was frequently taking them off to flip book pages and the like; now I wear them full time. For tools, I've always carried the issue orange MC-1 switchblade, which can be easily used if one hand is disabled, and a medium sized Swiss Army knife. I also have mini survival kit in a pouch and a Leatherman for my gun belt which is worn under our body armor. Although we have the full survival vests available, they are not worn full-time like the pistol belt. Most of us attach an iNOVA Microlight to the main flightsuit zipper pull. And don't discount having extra earplugs in a pocket for anything you fly ... they can be a sanity-saver.
At the airline, I'm stuck with the airline uniform. They require lace-up shoes and, although I always thought Wellingtons would be sharp, I see the value in regular shoes for an egress down the slides. I always keep an iNOVA Microlight in my pocket and my cell phone on my person, in case of power failure or in case we have to egress in a hurry (or both). Rubber, oil-resistant soles are about mandatory in my book, based on the water, oil, hydraulic, and de-ice fluids I regularly walk through. As a passenger, it's always long pants and shoes. My wife is probably sick of me mandating that but she packs her sandals for wear once we arrive. I marvel at the get-ups I see people wearing when they board. If we went down the slides, they'd be walking around barefoot and near naked, by my best estimation. Not that I have anything against that in some circumstances, mind you.
In GA aircraft, I've always been a fan of long pants and lace up running shoes. Some of that is safety and some is based on the AF motto "Dress to Egress". Even if I made a flawless forced landing in the boonies, I might have to walk out, so that has always tempered my choices. In cool weather I always wear a retired CWU-36 nomex jacket, and often leave it in the plane when I get to the FBO; it has my earplugs, pens, & sunglasses in it year-round. Have I worn shorts? More than I care to admit; that is a risk I accept. Laugh if you want but I always wear Nomex gloves flying GA regardless of season. I've seen first-hand (no pun), the results of not protecting your hands ... in an egress, they may be your key to getting out. The only other twist for GA is wearing a chute for acro. I don't have a GA-compatible helmet, but long pants and good-fitting shoes or boots would be a must. I think that between bailing out and opening shock, regular shoes could easily be lost, unlike boots.
Warbirds: I'd lean towards the military flight gear. It works and the look is accepted/expected for that type of flying. Shoes are fine unless you're in a parachute, and again, I see boots as a smarter choice: they will stay on, possibly help the ankles, and be there for your walk to a pay phone (if you can find one).
I guess I rambled more than I intended, but the truth is, a lot of thought goes into what I wear and what I carry regardless of the aerospace machine I'm about to fly/ride in/on. And you never know when one small choice will affect a large outcome.
Ken