I played a lot of AoE 2 and just started up an Egyptian civ in this version.
Once you get into the actual missions this hits similar beats in terms of gameplay. The artistic style is the most noticeable change at first glance. AoE 2 obviously used older tech(ie sprites) but also had a more realistic style and overall tone whereas this game uses a more "cartoony" exaggerated features look.
The way units are upgraded seems a lot different too. In the older games there was just a huge tech tree to move through and upgrades to units came from structures in the game such as an armory as you progressed through the ages. In this it seems like most upgrades come from equip-and-forget items... Interesting, but I feel like it will take away some of the feeling of accomplishment... I haven't actually got to play around with it yet in my own game, so I could be totally wrong.
Overall, I am enjoying AoE Online and looking forward to what they do with the free-to-play model.