The art style used in Age of Empires Online is quite different from the realism of previous games. The colors are bold and bright, and the characters and buildings have an almost cartoony, exaggerated look about them. There is a sense of humor about the design that makes it very appealing. There are all sorts of little touches that add charm to the experience, like workers who literally stand around and snooze when you haven't assigned them a task. You'll be rooting for your soldiers to win each fight, and zooming in on them to see the colorful animations when they fight the bad guys is quite fun. The menus and interface share a similar style, making Age of Empires Online a truly fine looking game, indeed.
The big question about a free to play title, of course, is whether the free play is more than just a crippled, demo-like mess that requires you to spend a lot of money to even enjoy it. This is certainly not the case for Age of Empires Online. I played through several levels before activating the premium content, and there was more than enough game there to keep me busy. A free account can still level up just as far as a premium one, you can select both civilizations in the game (Greek and Egyptian), you can go on many different quests, and even play in PvP and in co-op (more on this later). The game doesn't really feel restrictive to free accounts at all. So what do you get for purchasing a premium civilization, then? Access to special units, a higher tier of technology and the ability to equip and craft even the rarest of items. It basically gives you a few extra nudges here and there that will give your civilization small advantages in battle. If you want to be the king of PvP, you'll want these benefits, for sure; but for the casual player, or one who likes to play in co-op anyway (that sounds like you, dear reader!) it's not as essential.
Speaking of co-op, how exactly does it work? Many quests in the game can be completed cooperatively. This works in a couple of different ways. You can simply attempt to join a co-op quest via the random matchmaking feature, which may or may not work quickly, or you can invite a specific player to join you on a quest. I played co-op with Nick last night, and I was able to invite him with no issues even though he was far lower in level and was playing on a free account. The scenario was based on surviving for fiteen minutes, and with Nick's help, my town center was able to withstand the oncoming attackers. There were some downsides; Nick didn't get credit for completing the quest, nor any quest rewards, as he didn't have that quest to begin with. He did, however, get quite a lot of experience, more so than for a quest of his own level. I suppose it's set up this way to prevent power leveling shenanigans, and it's not a huge deal, but it is worth mentioning. Still, the co-op works quite nicely in Age of Empire Online, and its much better than simple comp-stomping, as is the case for co-op in most RTS games.
Besides missions that have the co-op option, premium players that download the Defense of Crete booster pack can play a Horde style survival mode. There's other co-op inherent content though - for instance users can initiate trades of items and goods between their capital cities.