It’s a pretty long list. For starters, MuseScore doesn’t support open meter, microtonality, condensing, DTP-style text frames, ossias, divisi, multi-movement works, harp pedal diagrams, cues, Insert mode, musical manipulations, etc., etc., etc. It doesn’t have anything like Dorico’s popovers, or jump bar, or time-saving input tools like inputting across multiple staves, or sophisticated tools for managing musical logistics. Etc., etc., etc.
You can think of MuseScore as being functionally equivalent to where Sibelius was around a decade ago, but with significant upgrades in terms of the quality of the default sounds (for the kinds of music for which they sound good), and some much improved algorithms for horizontal music spacing (though vertical spacing is still a weakness).
That’s a fantastic level of functionality for a free product, of course, but it’s got a long way to go in many areas before it approaches the level of sophistication of Dorico Pro. You can be sure the MuseScore developers will be working towards ticking things off that list, of course, but we won’t be standing still waiting for them to catch up with us. And they appear to have some serious software quality issues right now that will inevitably take some time for them to address (I took a quick look at their forum yesterday and there were more than a dozen pages of threads started in the last few days, all reporting issues of various levels of severity, and the vast majority of which have as yet had no input from either the full-time team or even the more prominent developers in the community).
It’s also worth spending a moment to consider the wider business model: all of that development work costs a lot of money, and the money doesn’t come from sales of the software itself. Plus, despite the fine words about the importance of open source software and how it can never be taken away from the community (true sentiments, with which I agree), the bits of MuseScore 4 that people are most excited about – the new sounds themselves, the sampler engine, the MPE system – don’t appear to be part of the open-source project, so they are subject to the whims of the parent company. How many other important parts of the product will also be created in a proprietary, closed-source fashion in future?