Floaty movement and stiff character animations. What we've been shown so far looks great for (what I assume is) alpha gameplay? The idle animations are nothing to shake a stick at, but I'm none too fond of the way Yooka jumps with no takeoff and his limbs sticking out in a strange pose. Banjo always controlled with a great feeling of weight and "oomph," especially in the way he jumped. His arms would swing with momentum and his landings would leave him sliding on his feet a bit. I think control and animation are the two key factors in making a game that feels good, and both seem sadly overlooked in most modern day games. I have faith that the Playtonic boys will do us proud by the time of release, though.
Ignoring sounds for soundtrack. I don't think we've heard any in-game effects beyond the amazing two musical pieces, and I'm hoping the sound design isn't too overlooked by the end of things. Again concerning the feeling of weight, I'd love to hear Yooka's footsteps pitter-pattering in a satisfying way. I can't stand seeing a character run around in silence, it makes them feel like an object being slid across the ground, rather than an organic participant in the environment.
My last comment is completely personal, but I'm really hoping the camera isn't locked at a far distance from the characters. As a kid I always played B-K, B-T and DK64 with the camera as zoomed in as possible, because it put me in the moment and made me feel closer to everything in the world. Today I find, especially in what few 3D platformers there are, the character is a speck of dust, and the level is treated like the real star of the show. The joy of exploration is sacrificed for a practical view of what you're meant to see. The designers want to account for your high resolutions and widescreen TVs by giving you a ridiculous bird's-eye view, and it has always been a turn-off and broken my immersion. I've forever preferred getting up close and personal with the details of whatever's closest to me, it's simply how I prefer to play these games, and I'm personally hoping (though not greatly expecting) that to be an option for me with Yooka-Laylee.
The floating islands are kind of bothersome, too. They definitely detract from the uniqueness of the scenery. They're a bit "Skylanders."
And, uh... Maybe give Yooka a little shirt? Totally subjective. I just think he'd be cute Winnie the Pooh style, yeah?