It is not if the Bitcoins we hold on exchanges are a significant part of out stash (which should not be for reasons well known) the question is if it will be a significant part of the exchanges stash.
Also and some people here have hinted it (even if backwards)... chain analysis. Yeah, but not of the users but of the exchanges. It will be an exceptional opportunity to clearly identify hot, warm and cold wallets of most exchanges in addition to what we already know. To quantify holdings and, more importantly, to extrapolate the impact on the cold wallets in relation to the average "proof of keys".
I don't know why some people think 3 january is the day they should do the withdrawal, that's crazy, it should be done way ahead. 3 January is the day where everyone wanting to follow should have already been out for the whole day.
Things like this maybe would push exchanges to implement a proof of funds in the way I outlined like one year ago. The fact they are not doing it already, being a very simple process to implement, makes me very suspicious.
For the first time in history we have some asset that can be cryptographically proved to be 100% non fractional and we are not taking advantage of that. Shame.