Assassin's Creed is actually a good game. I believe the only reason it is not praised is because people focus on the sequels and how much better they are.
First of all, the scenery is beautiful in this game. The world is big and spectacular, and pulls you in to explore. Next, the controls are very fluid, allowing you to free run across rooftops easliy. The combat is very good, allowing you to go bashing your opponent, or waiting for an attack, then countering it with a devastating, more powerful attack, most likely finishing them off. This game mixes lots of ideas very well, such as stealthing your way through to avoid attention, then busting out the swords to battle with mostly annoying guards. Finally, the platforming-like feel you get running around as an Assassin is undeniably awesome. The voice acting is incredible and the music gets you pumped and sometimes bloodthirsty, helping you become the mightiest Assassin of them all. The game went into great detail in everything they did, proving a lot of hard work was put into this game.
Now for the bad. The ending was not very good at all, confusing everyone. Fortunately, more light is shed in Assassin's Creed II, but those with a blown budget will stand by scratching their heads and wondering what the heck just happened. There were also some annoying glitches visually, and was not very proffessional from Ubisoft.
All in all though, Assassin's Creed was a good game.