In my mind, there are two kinds of series, theres a series with a good first game, and a bad one. some good ones would be, Halo, Diablo, Civilization(addicting, if you've ever played it)
Some of the bad ones, well most don't become a series if the first suck. Unless your Ubisoft, or someone with equal or greater money and power than Ubisoft.
AC1, (in retrospect for me) sucked. Very linear story for an "open-world" and the good bits got mixed in with the shovelfuls of horse manure you'll get from the game and its traveling sequences. But Ubisoft was set, they were gonna make this whole trilogy(possibly saga, because i thought the original concept for Ubi was for them two have 3 DIFFERENT games, not 2 and one remix) idea go. So they learned(haha a gaming-company learned, how rare) from mistakes, and to me released a pretty gem for me to rub my horse on(not euphemistically of course).
But the unfortunate side-effect of having a good first game, means your second will suck AC1's manure piles. And by the time the developers get to the third they will be soo tired of trying to fight for the to check a fanboy-free inbox, that they will give in and push your wettest dreams through their fingers (ewww..).
On the other side, a bad first game, means a good second, and then a mediocre third, and lots of spinoffs which will usually end with a series with a very large number on the end..
[small]oh shit, so i guess final fantasy just kinda jumped in on the "bad" train and liked it[/small]