The thing is when you do go to the other zones once you reach level cap, you do not see the other alliance in that zone. It is an instance of that zone created for the purpose of each alliance being able to do the content of the other alliances so they only players you will see when you go there will be other players from your own alliance. The content of the new alliance zone is ramped up so that it will be challenging for a fully leveled character but it is the same content you would do if you had rolled a character in that alliance only your will be doing it as a player from the alliance you started in.
What you do when you take on this more difficult content in the other alliances is you max out your characters skills, and eventually you can learn all the skills the game has to offer on one character. You can only slot 6 skills at a time but you will eventually have all the skills in the game to choose from. The level cap is 50, yes, and you have to be 50 before you can do this, but that is the way it will work.
Also ESO will have veteran points and I think these will also be what you earn when you do this 50+ and 50++ content.