I am a big supporter of always wearing helmets when out on the e-board, and most of the time on my conventional boards, always wearing mine (a standard skate helmet by Bullet). For once, I will leave out my usual diatribe on reckless fuckwit social-celebrities promoting helmet-less city traffic riding and just cover some options for protective gear.
I have considered a full-face skate helmet, with greater protection but lighter than a motorcycle helmet, and may get one depending on my riding.
UPDATE: I got a full-face Giro Switchblade MTB helmet, sadly one accident too late – I involuntarily came off and kissed a tree. That will teach me! Now I have the switchblade to save my pretty smile. I went with the MTB helmet as it was cooler than a full-face TSG and, for pure vanity, less conspicuous for me.
The other things I consider essential are my gloves, where I use normal cycling gloves as they offer decent friction protection without limiting remote-control movement.
In addition, when going fast (and not nice and friendly with my youngest son) I use my armored motorbike jeans made by Force Riders, with armor in the knees and, importantly, hips. Previously I have had a bad pelvic injury, so protection of my hips is important for me. Interestingly, pelvic protection is not normally worn by skateboarders, but as I have learned it can be a fragile body part if you fall in a specific way.
I am not sure if Force Riders jeans still exist (I could not find any links to them – mine were bought on Amazon), but there are other makes, including Alpinestars, Richa, Bull-it, UglyBros, Roadskin, and Draggin, who are well established and with a solid reputation. My Force Riders were relatively inexpensive, but I replaced the basic foam protection that came with them (same with many of them) with much better and more flexible sorbothane and shock foam CE rate armor. They work really well, especially when out just cruising around the local cafes (looking less kiddie-like than external knee pads), and I would highly recommend wearing them.
Following the motorcycle theme, if I want to go full protection for conventional downhill, or fast on the e-board, I use my British Knox under-armour under whatever I am wearing at the time and is not going to shred easily. Again it is light, very comfortable and works really well allowing good movement on the board.