Assassin's Creed is a game that was much more interesting before than after the release. As already pointed out, much hype, and of course the ingame footage looked amazing. However after it was out, people soon realised that beyond the visuals, AC was pretty shallow in terms of story presentation and gameplay variety. That, together with the lukewarm reviews hurt the legs, even though it managed to create a surprisingly big fan base, compared with other games with similar review scores.
/edit: about the used game sales: This actually could have quite a big effect. The combination of how attractive it looks and how "bad" it was reviewed probably left quite a lot of people unsure wether it's worth buying or not... so as a compromise, they bought it used. This idea is even supported by the fact, that it had such a big first week/month, but generally disappointed people - so soon after release, there were a lot of used, cheaper copies to get, because a relatively high percentage of the "early achievers" (which were many anyway) wanted to get rid of it.
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