The level length is actually very similar overall. TF definitely has some longer ones, like the swimming levels, but it also has a lot of levels that were shorter than the average level in Returns. So in terms of overall content DKCR slightly exceeds TF I think. Level design and difficulty, as you acknowledged, is debatable, but I give the nod to Returns in these regards as well. I thought the higher difficulty allowed them to be even crazier with the level designs. Secret exits were a negative for me. They weren't difficult to find and it added nothing to my enjoyment, forcing you to replay a level. Secrets were hidden in the exact same manner as DKCR, so I'm not sure what point you are trying to make there. I never thought I would miss the blowing, but it's replacement, the pulling mechanic, was severely underutilized and as a consequence, the game felt less interactive Returns. There were way less mine cart levels than Returns, less Rambi levels, and less rocketbarrel levels, all negatives for me.
The rest I can agree with you. I absolutely loved the water levels, the Different Kongs, and the overall variety. The enemies and bosses were MUCH stronger, and I don't even need to mention the audiovisuals. The game was amazing, and an early contender for my GOTY, but I'd rank it just every so slightly below Returns.
