It's similar to traditional Age of Empires in a general game play sense, but they greatly expanded the "RPG" aspect and made it the core part of the game. Your "home city" has a level that can be increased with XP (gained from doing missions and PVP I believe). Increased level gives you access to better units and equipment.
But, this is a free to play game, so you have to have a pay-to-win aspect. People who pay money get access to new (currently unreleased) civilizations, as well as booster packs (extra missions that grant new gear/equipment/bonuses).
As someone who didn't level up high enough to get my hands on expensive equipment for my troops, I am at a major disadvantage against someone who does have these bonuses (they aren't insignificant either, I saw +$40 damage for all infantry type bonuses and whatnot). Also if you want to just play typical AOE where you can set the game conditions and stuff, you have to buy that privilege from them.
The cartoony graphics work okay in games like Starcraft II because there aren't all that many actual human units on the map, most everything is a machine or alien. In AOE:Online the higher end units are physically much bigger than the standard units, so it looks kinda weird.
That's what I remember from the beta anyhow. It is a free game though, so everyone can see for themselves whether or not they will enjoy it.
Edit: I see several day 1 DLC addons, including "premium" packs for the existing Greek civ, which basically adds better gear/stats to your units. How on earth pay-to-win is supposed to work in a (supposedly) competitive RTS, I have no clue.