Everyone can be viable if you work them. There's no truly useless character. If you grind up Amelia, she'll be good. Eirika, she'll be good. Tana? She'll be good. I like that. It lets me pick my favorites instead of needing to use a character because they have the stats I need to win.
Also, I loved the monsters. I know they were taken from Gaiden, but I didn't like Gaiden, and it felt really good to see more variety in the monsters. I just wish there was a non-hacking/glitching way to play as them. I liked the Risen, haven't played enough Fates to get to the monsters. I wish there were more non-humans in the games as a whole. Laguz and Manaketes are awesome, and I need a Mauthe Doog dangit! The only thing I hated about the monsters was the Baels, as I am a bit of an arachnophobe.