It would be nice if the pen/pencil tool in the comment tool set didn't auto-smooth a fraction of a second after the pen stroke. I fill out some PDF forms on a regular basis which are then emailed. I love maintaining a paperless workflow but a lot of my writing is in small-ish print (approx 10-12pt) at moderately high zoom levels and I find at that scale that if I'm not very careful about stroke speed and technique, the smoothing feature modifies character shapes in bizarre ways. Periods (".") and the dots on i's and j's don't take unless I draw them as a little scribble. I would love to be able to write naturally with fluid strokes that render the way I draw them. This seems to be a very commonly desired feature so I wouldn't be surprised if it's been requested before (I searched and only found one request, which I voted for). A prime example of how I think the inking experience should be can be found in Microsoft OneNote. Pressure-sensitive variability in strokes would be a huge perk as well, bringing the experience even closer to traditional pen-and-paper performance.
Also, the eraser is buggy in that it takes several attempts to completely erase something I've written; first, second, and third attempts frequently leave little artifacts of the strokes which seems like very odd behavior. It should delete all ink (partial strokes) under the eraser stroke, or at very least delete every stroke (entire strokes) touched by the eraser stroke. I'm using a Microsoft Surface Pro (1st gen) with a subscription to Acrobat DC (version 2017.012.20098); considering the proliferation of these kind of devices I know a lot of people would appreciate these improvements.