Could be the bias of nostalgia, but I like this game quite a lot.
Biggest criticism I've seen is that it has repetitive gameplay. Your tasks revolve around you killing nine different targets, each one located in a different district of a city, accompanied by missions like eavesdropping, pickpocketing, flag collecting, and more. This repetition didn't faze me at all, because the end result from these missions is you get information that will help you assassinate your main target (# of guards surrounding target, how to get into the area, what the target is wearing, etc.). This level of investigation is something that I miss in the later Assassin's Creed games.
I'm also a huge fan of the story and the setting. The cities of Jerusalem, Acre, and Damascus are really well crafted and still look impressive today, in fact, the whole game still looks really good for 2007. The atmosphere, created by the music and the gloomy visuals, is really well executed too. And the story is done so well here, revealing the overall narrative bit by bit, introducing new concepts until the very end of the game, while also focusing on the protagonist's character progression.
But, there are just a few issues with this game (on PC). I ran into a couple crashes, the pop-in is abysmal, you can't really play above 60 fps without the game starting to act up, you need to do some tinkering to get your controller to work, and no subtitles (seriously?). You'll also find yourself screaming "that's not what I wanted to do!" sometimes, because the parkour system isn't always solid.
I've seen a lot of people recommending newcomers to skip this game and go straight to 2. DON'T. I truly believe this is well worth your time.