Edit: When Sim City went online only, I was fully expecting this game to be too, but not really F2P. Wasn't really looking forward to this one anyways, but it sounds as shitty as only EA knows how.
Age of Empires online is an RTS with a free to play model. You start with a couple of civilizations but you can buy more. I think you also reach a point where you need the advanced units which have to be purchased. Not completely sure on that one. Maybe someone who has some experience with the game can elaborate.
All hope rests with Company of Heroes 2 now.
The starting civs in AoEO are (very) limited. They are more like trial versions of the ones you buy with money. When you buy them, you unlock more missions, storage space (there's crafting and loot), equipping rare/epic loot, advisors, and the advanced units you talk about (can't be bought separately), etc.
They just recently modified the game to add micro-transactions, but it's mostly cosmetics for your units. The game is doing OK, but not great. The Single Player is somewhat entertaining, but there's not really much to it.