I don't know what you refer to as "story" for Ostagar but it did flow better than other DLCs I've played, certainly better than The Descent. After all, it made sense (to me anyway) that you would at some point return to Ostagar to give Cailan (and Duncan) a decent burial as a sort of farewell. If I remember correctly, you can also retrieve Duncan's blade. Witch Hunt also flows better in the sense that for my Warden who romanced Morrigan it was only natural that he'd track her down, but I agree it suffered heavily from recycled locales and could've benefited from your DA:O companions. Both these DLCs gave my character a reason to care, similar to what Legacy does for Hawke in DA2 by tying it to Malcolm.
Ostagar doesn't really contribute to either the over all story of Origins, or the meta story of Dragon Age. The loot is nice, dealing with Calian is touching, but a playthrough doesn't really miss anything if you don't go.
Warden's Peak is a short story about Wardens from the past, not unlike Jaws of Hakkon which focuses on the last days of the previous Inquisitor. Again, in both examples, there is a connection there for your character to care. It wasn't an incredible story but it was an enjoyable side quest.
For me the quest was take it or leave it. It was really useful only in demonstrating that the Wardens are end justifies the means types, but we got that with Ser Jory's death.
I fail to see what was so complex about JoH other than perhaps its use of the Frostbite 3 engine. Like Warden's Peak it's a story that takes longer to piece together because of the larger sandbox (it's basically a miniature DA:I).
Nothing to do with the engine and everything to do with the lore. The Avvar to this point were basically untouched. We get a new perspective on spirits and how the Dalish/Human conflict blew up with the Dales refusing to send aid during the second Blight. The fact there were mages in the Seekers, and Ameridan had serious qualms about the usage of the Rite on mages, because of the potential for abuse.
And of course the Inquisitor himself.
A number of these things connect to what DAI deals with. Plus it really is more attractive than any DAO area. And Swiss Family Robinson tree houses!
Okay, I did fangirl there for a moment...
My big problem with Witch Hunt, besides the reused areas that only vaguely made sense (I know what they said about Cadash Taig but it felt shoehorned in to escape creating a new area) is there was no urgency, and honesty it was overshadowed by another DLC that had a returning companion in Shadow Broker. I suppose it might have felt more urgent if Morrigan was able to react to the Warden based on approval, not just romance, but so it goes.