Exceedingly formulaic & repetitious, story, strategy, and even fun all are subservient to very archaic game design.
The biggest issue with D3's archaic design is that every mission is played out almost exactly the same way. You start with almost nothing, then recruit weak units, build your upgrades, and move along a linear path until the end. Then you lose all your cool upgraded troops and start over by recruiting weak units, building your upgrades, and moving along a linear path until the end. Repeat until you get really bored (it won't' take long).
One good point is the license board (like FFXII) for your hero units. This is a fantastic addition that attempts to bring D3 into at least the year 2006. But you know it's bad when the only thing to praise is a feature directly taken from a 2006 PS2 RPG. And why can't I have license boards for all my units instead of boring generic units? Sigh…another failure of D3.
Continuing with D3's many faults, the unit class progression system is exceptionally poorly designed. Say you have three knights. Rather than let you pick a class path for each knight, you pick a universal upgrade path so all your knights become either paladins or heavenly knights, for example. This makes no sense lore-wise or gameplay-wise. Worst of all, it would have trivially easy to allow unit by unit progression.
Ultimately, if you strip away the very nice 3D graphics and artwork, D3 is really an incredibly shallow game. For a "strategy" game, there is almost zero strategy. Every battle plays out almost identically to all others, which doesn't bode well since you have to fight many, MANY battles. The addition of certain double-damage spots on the battle map makes the strategy go from infantile to perhaps juvenile. Again, is this something to celebrate?
The entire time I was playing I couldn't help but realize how VERY similar D3 is apart from the battles to the Ogre Battle games (for SNES, PS1, & N64), or even Dragon Force (obscure Sega Saturn game), except D3 is vastly inferior in its army management, class system, story, characters, map design, ect. And about the battles, there are so many more strategic games like FFT and nearly every other TRPG. Why D3 couldn't steal some ideas from these games, I don't know.
To end on a good note, playing D3 did remind me of how awesome games like Ogre Battle, Dragon Force, and FFT are. In fact, it has made me want to go play them. And chances are most of you haven't even heard of Ogre Battle. Do yourself a favor, go play Ogre Battle for the PS1. At least in this D3 will have inadvertently done some good for the gaming world because in itself it's just rubbish.
High points: Great graphics and animations, license board for heroes, Lambert is cool (Praise the Lord!! line is hilarious)
Low points: Everything else, too slow pacing, most out-of-place narrator ever
Final score: 3/10 "Play only to remember what a bad game looks like"