I've taken up a new tradition of playing exactly one Assassin's Creed each year. Seems to be a good pace since I didn't feel bored with Brotherhood at any point. Dunno what will happen with Revelations, but I will be playing it next year. Nevertheless, it is a surprisingly good series. The first game went wrong in many ways, sure, but the sequel and Brotherhood are both excellent examples of good game design on many fronts. Let's take a look.
1. Empowering the player
Assassin's Creeds are sandboxes and the player is almost omnipotent within. The protagonist is a master assassin with all the appropriate skills. What's spectacular about Assassin's Creed is that the designers have found a way for the player to truly feel empowered by each and every one of these skills. Every trick in the master assassin's bag is at the player's fingertips. Commanding Ezio to do all these things is as seemingly effortless for the player as completing the task is for the assassin. This is achieved through a lot of helpful automation. The marvel in Assassin's Creed is how well all this automation has been implemented as it rarely makes mistakes. The controls are not perfect, but they are quite close, and most importantly they are really powerful.
What's achieved by these powerful controls is that the player is empowered to truly take upon himself the role of a master assassin. Execution is never hard, but the planning is, and that's what's largely left for the player to figure out. More importantly, the player is capable of constantly doing awesome feats and can decide for themselves which feats to use where. Perhaps the prime example would be Ezio's ability to instakill enemies while attacking undetected. There are many different scenarios to do this. The simplest is to walk behind and enemy and stab them with the hidden dagger. Or a pair of enemies once dual hidden blades have been obtained. Ezio can just easily kill enemies by dropping on them from high above with perfect precision. The latter feat is achieved quite easily in the game because Ezio always has a target if there is one within range. There's no need to lock onto enemies separately (although that can be done to make the camera follow them).
All the systems in the game follow a simple control scheme which is really easy to learn and does a lot of things with the press of one or two buttons. The manual dexterity requirement is really low - Assassin's Creed is the thinking man's action adventure game.
Combat also follows the rule of empowerment through effective controls. What's remarkable about combat in the series is that while it's not very realistic, it feels right. Once again, execution is easy but effective fighting requires planning and careful positioning. Ezio can easily dodge or even counter enemy attacks. He can lay waste to multiple enemies in a kill streak. With all this power, he still loses very quickly if the player just attacks mindlessly. The combat system is all about waiting for an opening, followed by quick retaliation. Ezio moves gracefully in combat, and automation takes care that fighting looks cool. More importantly, it feels cool. It is also fairly slow-paced. This means that getting into a fight is always a delay. Although fights are rarely difficult and often very satisfying to fight, it is often at least equally satisfying to avoid them by silencing enemies one by one.
The most important change in Brotherhood over the previous title is that the secondary weapons are now actually usable in combat. Depending on what main weapon Ezio is using, holding the attack button makes a secondary weapon attack. Still, the game has a lot of weapons but swapping between them is quite pointless. This I think is one thing the developers should look into. Pretty much any weapon can handle 99% of all the situations. With swapping weapons being a bit too clumsy, this leads to a lot of things that largely go unused.
I do have one complaint about horseback fighting. The game now features horses as an important means of travel, and they can also been seen in combat. However I found it a bit weird that attacks from horseback are really, really weak. In most cases it is much faster to simply dismount and take the enemies on on foot than try to whack them from horseback. If horses are involved in later installments of the series, I hope the developers give riding attacks a whopping damage boost. I want to ride through my enemies and absolutely smash them.
3. Lots to do is not content unless it's also fun
This is an important lesson for sandbox games. Giving the player freedom do a gazillion things is not enough. What's really important is to make every single thing that the player is able to do fun. This is the lesson Assassin's Creed developers have taken to heart. The Rome of Brotherhood is full of things to do and they are almost always guaranteed to be fun for the player. Hell, even starting an entirely pointless fight can be fun with the game's combat system. Sure there are always some misses in the huge amount of side missions available, but they are in the marginal. The takeaway is simple. There's no point in including a task in a game if that task is not fun to perform. There's also no point in repeating a task that gets boring easily.
Assassin's Creed: Brotherhood is a must for anyone involved with action adventure design. So many things have been done right that it's hard to count all of them. Could be that things are even better in Revelations but I haven't checked it out yet. I'm more curious about Assassin's Creed III though, since it is finally a completely different setting and should take the series into an entirely different direction.