I see Beef posted but I am going to post also just because.
For me D3s strong points lie in:
His impressive grab range. Tied for best non-tether grab range with
His weight and very high durability. Combine this with his fast fall speed and he is the HARDEST character to kill off the stop.
He has a strong zoning and spacing game.
Chaingrab gives him good damage racking and possible follow up. Tech chasing on non CGables still offer us a positional advantage and the chance to punish hard.
He has pretty legit killing power as well.
His cons are pretty numerous though:
Gets combo easily due to his weight and fast fall speed.
Has no real projectile and gets camped pretty hard.
His recovery covers a good distance but is REALLY punishable. If you get predictable you get ****ed really hard. Especially against MKs. Its a stock if they do it right.
He's pretty damn slow.
He's not good at approaching at all.
His overall moveset is really limited and requires the player to be very creative and proactive in their reads.
His killing moves are incredibly slow or telegraphed. His only reliable one Bair will almost always be stale when it comes time.