I spent 6 years in the Navy in real life and don't have any interest in being in a military in a video game. I do intend to be a blacksmith on my main so making swords, spears, and armor could be a fun diversion from the srs bzness of making nails, horseshoes, hammers, and plows. That said, if the combat is realistic and historically accurate tactics can be implemented successfully, I might change my tune. If it's just large scale PvP like you would see in a fantasy MMO, where 80 people are standing around clicking fire on their siege weapons while 30 other people are dropping endless hails of arrows on them from atop a keep, I definitely won't participate. Especially if something like military discipline is expected.
If cutting down the horse of a charging knight can break his arm and then he has to fight his way back to reinforcements, only to have to walk 28 miles to buy a new horse, and shield walls are a thing, and good armor means that you only take damage from guys with big heavy blunt weapons or thrusts to the gaps I'd be intrigued. If it's a battle of 'I have 30k health and do 15k DPS and you have 50k health, do 5k DPS, but have heals' I don't care about it at all. There are already games like that, and I know I'm mediocre at PvP in them. If I'm going to be convinced to take part, it will require something that I don't think has ever been done except sort of in RTS games.