Well thanks for the replies but I want more!
That can be arranged 
The only "real combat" I've ever been in has been the kind with pads and gloves, so I honestly have no idea what "real combat" is like. I'm approaching RCM as simply another way of playing a game, and am asking from the gamer's point-of-view what the game-differences will be.
You've answered that somewhat: poorly-armored troops are cannon fodder. Got it.
And even cannon fodder has its effectiveness. You need something to empty the quivers of those nasty Elves before engaging them with your better troops. Is horse archery still a counter to Dunlanders and Isengard?
Yes.Is cavalry still devastating against archer-heavy companies of Corsairs?
Yes, as long as they are armoured. Will a few top-tier units still wipe the floor against a large mob of yard trash?
Generally speaking, no. Getting swarmed will get you killed by a thousand little cuts. Fighting anything largely outnumbered is very difficult.How different is an RCM game versus a normal game?
Combat is generally speaking more deadly. You will be able to take less hits and deal a lot more damage to opponents. Good use of terrain and tactics are very important. Armour and weapon choice matters a lot. Armor piercing weapons are reduced to what historically should have been armour piercing.How differently would I need to play my own character in order to succeed? (Like can I ride out solo to harass the enemy with horse archery before the two sides clash, or will I or my horse inevitably get one-shotted?)
You should expect heroic solo charges to be a quick ticket to a grave. You can't expect to do horse archery by yourself against heavy archer armies or you will probably die. Unless you're riding a very heavily armored horse. I have used Rohan horse archers quite effectively against Dunlanders and Isengard, but when I send out too little they will all be slaughtered by archers and throwing weapons.