To be required is to be essential or absolutely necessary. It seems in today's races, that the only thing essential about an item on a required gear list is that you need it to pass inspection before or during the race. The required gear list has manifested into a mix of items that truly are needed in order to participate, that are necessary by law, that might be handy and those that promote safety. Based on these categories, it is my belief that we can do away with the required gear list and proceed to a future with more streamlined pre-race proceedings and more room in our packs for stuff we actually use.
First let's consider equipment that truly is needed in order to participate. These might be mountain bikes or boats. We don't need to list these items. When a race lists mountain biking or paddling in the disciplines it's strongly implied that you'll need a bike and boat. For specifics, the rules can indicate whether cyclocross or road bikes are permitted; likewise for kayaks versus canoes and any paddle type restrictions. There is some grey area when it comes to races with a ropes section since the bulk of participants may not be familiar with what equipment they might need for such a thing. In which case, a concise note without the formality of a required gear list will suffice.* A special note for race directors: in your rules, be sure to include something requiring teams to have their bikes / boats with them when completing the appropriate sections. Advantageous position can be achieved by leaving your bikes / boats on course and finishing on foot for example. A 'no gear drop' clause can also cover this.
Secondly we've got the gear that is necessary by law. These would be bike helmets, pfds, a bailing device and tow rope etc. These can and should be covered in the rules. "A helmet must be worn at all times while on the mountain bike" means a whole lot more than the required gear list's stipulation that you have the helmet with you. Other items required by (Nova Scotian) law are matches, compass and a knife. A gentle reminder in the rules can make sure participants will carry these items as well.
The next required gear list items to discuss are those that might come in handy. Dearest race director, I do love you for what you do, but you're not my mother. And if it makes you feel any better, I wouldn't listen to her either. Yes, spare socks might be useful for some participants, but I prefer to make that judgment myself. I also prefer to bring the bike repair tools that I deem appropriate. And if I want to catch some color during an adventure race, it should be my prerogative. Now, I will concede that since I've been racing for 10 years, that I'm a qualified participant to make these judgments. However, if the purpose of your gear list is to make sure that new participants have all they need then you'll fail at that purpose. If your race will have new racers, then you should have some kind of resource available (even a link to a website will do) that identifies typical gear used. These types of items can be suggested and advised as much as you like, but not required. The best resource for helping new racers in this department is to put them in touch with an experienced racer; as an RD you'll have enough correspondence to worry about. Give them my contact info if you like!
Lastly we have items that promote safety. Adventure racing is ridiculously dangerous. A raincoat, toque, some gauze and a triangular bandage will not change that. It's always been my theory that any injury I could treat with the meager required first aid equipment isn't worth treating in the first place. Any injury requiring treatment, in my opinion, needs a whole lot more... including first aid personnel; will they fit in my pack? Sure I know first aid, but I can barely state my name after 24 hours let alone practice sound first aid! Again there is some grey area here with a raincoat and toque whereby hypothermia is a considerable risk in any race. However, the consideration for these types of things with respect to liability is slowly changing toward less requirements equaling less liability. When requiring an item like a rain coat that has no standard for qualification, like a pfd or bike helmet, the race director actually puts themselves in a liable position by qualifying that item. Take note of that one RDs; protecting your participants may not be protecting you.
If you're brave enough to host an adventure race, it's your right to include a required gear list if that's what you need to satisfy your level of comfort. But please, if you do include one, do it because that's what you want and you believe it's necessary for your race, not because it's always been that way, or because you think people will show up without shoes because they weren't listed for them.
*For races with a ropes course I'll go against my usual advice and suggest that there should be a pre-race inspection of gear. My reasoning: the prussik, for example, is often used as a redundant safety device. Unfortunately most people don't realize that your prussik can't be 4 ft long because it will either get jammed in your figure-8 / ATC or it will be too high for you to operate if it does engage. It's a very annoying thing to be in a bottleneck at a ropes course while the rope staff have to reconfigure someone's gear on another team. The same goes for harness adjustment.