AoE Online isn't really what I'd consider MMORTS :/ And as a side note, imo, the game is far from being an innovation... it just added grinding to the rts genre and is pretty much pay2win -.-
It has no persistent world where you actually play. When I imagine a mmorts I imagine having a persistent city and/or army. In AoEO you just have your city that is infact just your technology tree that other player can "visit" and send you items. The actual gameplay (which is like old AoE games) takes place in single games either against an Ai or another player... like any RTS game. The game introduces levels and items, when you complete mission you get levels and levels unlock the tech tree. You also get items with +% stats that gives you bonuses to your units when you build them. If you don't buy you cannot access the good items for your civ, and each civ is (was ?) 20$ each...
Levels and items are terribly grindy to get and pvp is a joke since you stomp lower level players and players that have not paid.
That game was such a fail (at least when I tried it)... where are the ages of AoE and AoK...