I got no problem with linearity. I often prefer linear games. The thing I hated was the story, the characters, the combat system, the level progression system, the generic-feeling visuals, the hilariously bad dialogue...etc. I genuinely do not understand how anyone can praise the paradigm system (especially not when also bitching about FFXII's gambit system).
Ff13 is easily one of the best combat systems in an ff game, of course that depends entirely on whether you use the active mode from previous games or stick to the default passive modes.
The problem with ff13s combat system is that it only ever gets a chance to shine once you have 3 members in combat, and actual resistance from enemy encounters (like those boss side missions).
But yeah, a lot of the stuff around it was just not great.