The Official Nintendo Magazine had some time to sit down with Retro CEO Michael Kelbaugh and Nintendo producer Kensuke Tanabe, who were both responsible for the creation of Donkey Kong Country: Tropical Freeze. A number of interesting topics were brought up, from the fact that Iwata specifically requested that Retro Studios "spend a lot of time" working on the heroes' fur effects, to the general increased quality of the game.
Kelbaugh specifically notes that pretty much everything was overhauled graphically, and the addition of David Wise as composer was a major improvement. Tanabe also talked a bit about the Super Guide, and now players actually did not like the system, and wanted to clear levels on their own -- but the difficulty hasn't been sacrificed as a result.
Kelbaugh also describes a "symbiotic relationship" between Retro and Nintendo's numerous studios, and how they worked on Mario Kart 7 and Tropical Freeze at the same time. There's a bunch of other little tidbits, so I suggest reading the full interview.