Its sales are always monstrous yet alot of people seem to hate it with a passion. 

Its phenomenal popularity compared to other console jRPG games pisses a lot of people off. As they dont get why millions prefer FF over "better jRPGs".

Changing the platforms (N64 -> PSone, multiplatform 360 move) encourages the hate even further.

QFT I couldn't have said it better.

The repetition since FF7(Young Boy saves Da Warudo) and FF7 spin-offs are really to blame.

Sub in Wii Sports or Wii Fit or Mario Kart Wii or Halo or etc. or etc.

Success breeds haters.

There's that one saying: "you know you're doing something right when people hate you."

Onyxmeth and steamboatwillie are right.

And for the record, I'm one of the bitter old assholes who couldn't make the switch from the 2-D glory of FF6 to the 3-D cutscene snoozefest of FF7. The shock was so great I developed this horrible complex where I prefer all my RPGs, even new ones, in 2-D, or with ASCII graphics.  I can relate better to @ symbols than 3-D models now.

I think FF is still really good at what it does, I just don't like what it does anymore.

Same here, although I prefer FF4 (better music).

I think many of the haters simply want to make themselves stand out from the crowd of masses that love the game.

FF7 was fantastic, for me. (And many, many others.) And let's face it, it's probably the most ground breaking RPG since Dragon Quest 1.

Spurned Nintendo Fans?