Well the three very stupid reasons I always hear are.
1: It's popular. As I believe I've said on this forum somewhere before, liking something because its popular is stupider than sticking fireworks in your urethra (which I have seen someone do). There is zero logic behind this reasoning.
2: It has a bad fanbase. It's a known fact that there are heaps of awesome franchises have horrendous fanbases. Sonic, Star Wars, One Piece, Evangelion, Pokemon, Dragonball Z, some of the Final Fantasies, etc. This fanbases have no effect on the franchise itself. If you don't like them, ignore them, and enjoy the series itself.
3: It's too long. I hear this one all the time, and I just get my head around it. If you like the series then there being more of it is a good thing. A very good thing. I'm glad I can watch hundreds of episodes of shows like One Piece and Yugioh, rather than have all the fun gone in a couple of hours like with FLCL (don't get me wrong, I love FLCL too) And it's not like there's any rush to get through it, you have all the time in the world to watch these series'.