I would also like to add that it is normal and realistic that different cultures or factions exchange technology, be it through a purchase or a trade. It may even be stolen.
The point is that it seems to be one-sided for Starfleet. Rick Sternbach admits that it was a mutual exchange with the Klingons, yet there is no in-canon admission from Starfleet for using Klingon tech. Starfleet tech is usually presented as their own. Are they embarrassed?
Another point is that it's TNG style nacelles and bussards, and that's what it gets associated with.
It's been long enough since the last TNG episode that I'd like the Starfleet nacelles on Klingon vessels to be replaced by something else, especially with the new alliance between Klingons, Orions, Gorn, Nausicaans, and Letheans. I also happen to like the original look to the Klingon nacelles.
I suspect it won't happen, however, because Cryptic doesn't like to work on KDF.
I'm confused to the issue here, the newer klingon ship designs by Rick Sternbach that are canon show nacelles with more prominant grills and bussard collecters, therefore this is more a norm on Klingon ships now, it feels like you are confusing and mixing the 'Sci-fi canon world of Trek' with the 'real world production side' of Trek. As for what Starfleet may of achieved from the Klingons? I'd certainly say that technologies like Ablative armour of the Defiant would be amongst them, also the Torp launchers of more modern starfleet ships look more similar to the Klingon style ones as well, besides, whats more important is the technologies that are inside the ship, not the actual ship interior itself.
Eitherway, I really don't see that big an issue with the Klingon ship designs in STO (atleast not with the newer ones). It would be ofcourse be great to see more variation though!
Was the development of the phaser cannons in the Defiant inspired, if not assisted, by Klingon disruptor cannon research? I realize they're completely different systems, but the projection should be somewhat similar, I should think.