The Assassin's Creed is: Stay your blade in the presence of the Innocent. Hide in Plain Sight. The most important, Never compromise the Brotherhood. These are the principles you are to use to kill the Knights of the Catholic Church. (Specifically the Templar Knights.) With a nice Sandbox gameplay style, and a variety of side-quests there is much you can do. Save citizens from Knight bullying townspeople, find all of the flags hidden throughout the entire world. If there is a building there is a way to climb it. Don't feel like continuing the story, go around and kill random guards... Your world, mostly your rules. Although the game is fun, I can't agree with many principles they teach. Specifically, the phrase, Nothing is true, Everything is permitted... But the game was a well done open-world game by Ubisoft Montreal. 

Graphics, the game has really good graphics. The game likes to boast the next generation of graphics for games. Although it was next generation, I still had problems with the clothing interacting with him running. I found that occasionally I ended up in a building that was completely closed off, and I was falling. That didn't happen often, but nonetheless it makes the graphics get a worse rating than it should. This is the game that started out the famous, or infamous, series of Assassin's Creed, and they did a good job with graphics that are hard to beat.
For the graphics I gave the game a high rating, but not too high due to the failure with the leg going through the clothing, and the glitches I accidentally found. This game gets a 7/10

Sound, this game has really good sounds. The blades have good sounding when you draw them. The arrows sound like they are flying, and even running makes an accurate sound. The musical soundtrack for this game is really good. I truly enjoyed the sounds in this game. The voice acting is also really good. I did find that sometimes the music could get a little old, but for me, I didn't find it that way. 
For the great sounds in this game, I give it a 9/10. They had a good soundtrack, voice acting, and accurate sound interaction with the environment.

Addictiveness, this game really has an addicting feel, if you can get into the story. It has an intriguing story, much to do in the Animus, and outside of the Animus. You can go and ride between the three major cities in the game, and even venture into the small villages. Find all of the Knights Templar and kill them. And with 60 of them, finding them can be a challenging task. Go to all of the vantage points to find a place to rescue a towns person, or just go up there to find your main quests. There is really much to do, and if you want a complete experience, this game will take a long time to beat.
Because there is much to do, I give the game an 8/10. Although there is much to do, sometimes it gets a little old to go after the same stuff over and over.

Story, the game has really good story elements, but it can get a little odd, as the Assassin's Creed games are known for it's conspiracy theory. The Templars are behind every bad event in History. The story stays close to actual history, but changes certain elements to account for their conspiracy. I found it to be well done, and they did do a good job of telling it. So for that I give it it's well deserved rating.
The Story is really well done. For that, I give it a solid 10/10.

Depth, the game has so much you can do. Jump on a horse and ride around for a few hours, collect flags, assassinate random guards. Just on a building. Rescue innocents, and fight Templars, and you have the really good element of the story. But because I think I already said what you could do earlier in the review, I will not go into detail here.
Because there is A LOT you can do, I give the game a 9/10.

Difficulty, although the game starts out in easy, it gets progressively harder as you go along. And for that reason I gave it a medium rating. I find that it was almost too easy at the beginning, but I found the combat got really hard, very fast. There were parts at the end, where I had to fight the same people 5 or 6 times.
Although the game starts out very easy, it gets harder and harder with each Assassination. So, I give it a 7/10.

Overall, this game is fun to play, has a good story, great sound and much to offer, and that is why I give it the rating of 9.5/10.

The problem I had with the A.C series, especially the first, is they systematically failed to keep the game fresh.
The first 1 hour is the same as the next 10. Contracts follow a similar pattern, and the weapons all feel the same.
Not until Brotherhood, and especially black flag, did I feel the games stayed enjoyable.

1 certainly had a lot of good features, graphics are indeed good, although I wouldnt dock points for glitches. Not from graphics anyway. As for the legs through clothing etc that is i common problem in most games. Especially when multiple outfits come into play.

The sound was pretty flawless too.

I feel the addictiveness is overstated, but agree with your points. Frankly I'd give it a 6 there.
As for the collectibles, to my knowledge you gained nothing from completing them which was something of a bummer.

Story, maybe 8. Its certainly original, but again I feel the later releases did a better job of developing that area.

Im not sure about your final rating, you gave one 10, the rest 9 and under. Seems 8, maybe 8.5 at a push would be more accurate.
It was a good game for its time, but even back then I'd only give it around an 8. Now I'd give it a 6.

This is also a series I dislike, but this is a good review.