The Deep Roads were hands down the worst part of DA:O, only long terrible slog with nothing except combat. Devs better avoid making anything like that in DAI.
I don't know about that. While at the beginning it was just tedious fighting, the deeper you went, the more you felt that you were going to find something dreadful.
This section was slowly building tension, while your party was descending lower - from almost clean, ancient dwarven structures to absolutely hellish landscape, filled with darkspawn filth, dirt and horrible creatures. The atmosphere was getting darker and darker, till the grand finale, which was the fight with Broodmother.
It was in the Deep Roads, where you really discovered the face of your enemy. While before that, it was hard for me to treat the Darkspawn differently from your generic fantasy bad guys, after this section I realized how truly dreadful they are.
And all that important information was not delivered through dialogue or some laborious exposition - it was witnessed by the player first hand, with just several sentences. Telling the story through gameplay (and not through dialogue) is a good practice, and I actually hope to see more of that.