To me DD's story is serviceable, just there to provide a background for the gameplay to shine - fighting huge monstrous creatures, exploring and finding unique weapons and armors just to mention a few was what kept me in it, a bit of a similar feeling that I get when playing Monster Hunter, only with a huge seamless world rather than a few staged separate biomes.
As you advance through the game, newer, more powerful abilities become available, some of them you can't even afford the first time you unlock them due to the skill point cost, so when you finally unlock them it feels good to go out and try them out. Different skills are useful for different situations, which also incentives to keep trying things against different enemies.
And then the fact that you can switch Vocation almost seamlessly (and in fact, some unlock as you get more levels in "basic" Vocations) kept the combat always fresh and fun. Sometimes I got an amazing unique bow, I switched to archer to try it, if I found an incredible Two-handed sword, I switched to Warrior and so on. Spells also got more fun and powerful as you progressed through the game, some skills and abilities could only be found in dungeons or hard boss fights, etc.
I definitely wouldn't have played it for the story, in any case.