Anti-Climax Boss: Due to the game's mechanics, the fight with Robert De Sable can end quite abruptly if the player manages to knock him down early on in the fight and follow it up by stabbing him with the hidden blade as he lays on the ground. This can be done in a matter of seconds after the fight with him starts, effectively having Altaïr curb-stomp the man who curb-stompedhim at the beginning of the game.
Awesome Video Game Level: For some, the last level is the best level since you finally don't need to bother with stealth and just go all out Rambo style, with Robert de Sable and Al Mualim as the final bosses of great challenge for you to fight.
Breather Level: Between the action-packed assassinations, Altaïr has to complete multiple relatively peaceful investigation missions.
I killed them because I could, because it was fun! Do you know what it feels like, to determine another man's fate? And did you see the way the people cheered? The way they feared me? I was like a God! You'd have done the same if you could. Such power!
Disappointing Last Level: Most of the game is built around stealth and free running in sprawling cityscapes. The last part is a linear, rigidly structured sequence in which you have to fight your way through dozens of opponents with no possibility of evasion or escape.
Game-Breaker: Counter attacking with the hidden blade is a One-Hit Kill, even against the final boss (once you jump through his other hoops)... so long as you have the timing down enough to succeed consistently (key word here).
Not to mention, if you get the timing down well enough to consistently counter with the Hidden Blade in this game, you've just developed a skill that will make the entire rest of the series MUCH easier.
You can kill entire armies with the hidden blade. Throw two people to the ground and knife them, and others will start to recoil in horror or distract themselves. The ones who recoil can be assassinated before they recover, that assassination will make someone else react, and so on and so on...
Throwing knives can, especially in 2 and Brotherhood, be countered themselves, so a counter attack won't always kill, but throwing knives cannot be blocked. Even during a fight where the boss will block all your other attacks, throwing knives can make quick work of them.
The guards can feel like friggin' bats when you're trying to run and hide, and they pelt you with rocks as you climb away, or the alert goes off again just as you're about to dive into a rooftop garden. They also like continually spawning just after you've killed a bunch of them to save a citizen, immediately starting another fight.
There is also the beggars, who constantly harass you (and only you) for coins, and the lepers, who shove you (and only you), sometimes into a group of guards or an informer's assassination target.
The archers. Up close, they are just normal guards, but at a distance with their bows out they are unblockable. Throwing knives can easily kill them, but early on you only have five, and you have to either steal from thugs (whom you have to leave the rooftops to find) or return to Al-Mualim to restock them.
After one of the main Assassination targets used poison on some civilians, Altaïr makes a side comment to another Brotherhood member that poison is a coward's weapon. In Assassin's Creed II, one of the main character's weapons is poison (A blade dipped in poison, to be exact). Making it even funnier, said blade was invented by Altaïr himself, as Da Vinci builds it from the designs in the Codex.
Many fans of the first game call Ezio weak because he uses dual hidden blades and his hidden gun. "Altaïr never needed those." Considering that it was Altaïr who invented them... well, what does that make him?
It's the Same, Now It Sucks: Probably the most glaring criticism for the first game is how repetitive each of the assassination missions are. Each following the same formula of having to do the same types of investigation quests, which leads up to Altaïr infiltrating the area where his assassination target will be.
LGBT Fanbase: Malik and Altaïr. Strangely, due to the huge amount of Foe Yay between the two that lasts until about halfway through the game when Malik forgives Altaïr for costing him an arm and getting his brother killed. Though quite a bit of the resulting fan art set out to only tamely fill in the blanks of what Altaïr does when he rests at Malik's Hideout◊.
Memetic Badass: The In-Universe mechanics indicate that only a perfectly played game is canon, in turn implying that Altaïr was so badass that his enemies literally couldn't touch him. Keeping in mind that Altaïr is forced to fight dozens of opponents at once to the death at certain points. This has not escaped fans' notice.
The Memetic Badass page has more examples of Altaïr's awesome, but the best relates to desynchronization. The player (as Desmond) "de-syncs" with Altaïr if the player does something Altaïr doesn't remember doing. Being hit causes de-sync and misjudging a jump and falling causes de-sync; Meaning Altaïr was never, ever hit and performed Le Parkour perfectly every time.
Nausea Fuel: One of Altaïr's counter attacks with the short knife is an elbow to the stomach and a punch to the face, which leaves the victim standing there for a split second, dazed. You have the option of letting him fall and get back up again, or pressing X/Square to finish him off. If you finish him, Altaïr takes his knife in both hands, raises it above his head, and stabs the guy in the skull. Just... the sound effects (sounds like wet wood snapping in half) and the fact that Altaïr has trouble pulling the knife back out again... ugh. Or you can stomp on his knee, folding his leg in half with a sickening crunch.
Slow-Paced Beginning: It takes quite a while before you finally get past the exposition and get to work assassinating people.
Robert de Sable. Naturally the guy that curb-stomps Altaïr at the start is going to be a pain for the player to defeat.
Sybrand. Hop from boat to boat and prepare to miss jumps and drown or simply drown because the game decides to kick you off that surface you just jumped towards.
Robert aka Maria in Jerusalem counts as well. There are archers stationed around the cemetery that you can't take out. You actually have to run away in order to survive (unless you can dodge arrows and the highest tier mooks at the same time) but doing that will lead you through more guards than you've seen in the entire game to this point. It's not uncommon to have to kill 50 guards before you can even get a shot at your target and if you only mess up one hidden blade counter kill you'll be down half your health. It's quite frustrating.
Uncanny Valley: The generic character models have simplified hands, the fingers stuck together in pairs. It leaves them looking like they have two broad fingers on each hand, and never stops being disturbing. Apparently, the Animus can't render them like the main cast, or Altaïr can't remember any more about them than that.