Here's how Assassin's Creed works out in practice. Your master in the Assassin's stronghold gives you a target. You travel through the Holy Land to the city where the target resides, and make contact with the local Assassin's bureau. You then have to find information, but first you have to climb up to some high points so that you can get a wider view of the city and how it operates. After a few pinnacles have been reached, you need to get your data. Some of it you can get by watching then pick-pocketing those in the know. Other bits you can get through eavesdropping or by roughing up local insiders. Yet more scraps can be bought from brother Assassins, at the price of performing some helpful task for them. Once you've pieced a plan together, you go back to the bureau, then embark on the assassination attempt. Get your target, make your escape, and it's time to return to the master for your next objective.
Played through once, it's highly entertaining. Each component part works reasonably well, the climbing parts give you a chance to scale the most interesting parts of the city, and while not every assassination seems as cleverly designed or dramatic as you would have hoped from the various teasers, the murder and escape is usually thrilling stuff. The second and third time it's still quite exciting. Each new city has its own distinctive look and feel, and new weapons and challenges get thrown in. The problem is that when you repeat this same basic cycle six or seven times in a row, it all begins to get a bit stale. By the time I got halfway through the game, my interest was waning dramatically.
In fact, Assassin's Creed has the same basic problem as this year's much-loved Crackdown: it can feel like a whole game composed of side missions; the core missions seem to have been mysteriously pulled out. Crackdown got away with it because its rather scrappy missions - not to mention the various mechanisms of exploring every inch of the city and developing awesome super powers - were so much fun. In Assassin's Creed, that isn't always the case. The core missions are a bit samey. The actual side-missions, which involve collecting flags, knocking off templars and assisting victims of guard brutality, are okay, but not actually that compelling. Even the Holy Land outside the cities is lacking in fun. It has a few towers to climb up, a few flags to capture and a few gangs of soldiers to defeat, but it never comes alive in the way that, say, Hyrule does in the Zelda games.