I've been an avid evangelist for MC for years, but there are a few key ways it doesn't compare well to various competitors.
JRiver doesn't seem to know who it's market is. This is important because the software needs to address the needs of it's users. Is it aimed at audiophiles building high-end custom systems? Is it aimed at iTunes ex-pats looking for a better/deeper experience? Should it be courting casual listeners who just want an easy program that works? For any of these markets JRiver has some big strengths and some big flaws.
It probably appeals the most to the audiophile, however the overall experience of using JRiver is not nearly sexy enough. The interface has this clunky, dated look. It should feel sleek and sharply designed so that it feels like something special. The audiophile user should know, before they use a single feature, that MC is the hottest piece of software available. Right now, when I launch the program it has all the sex appeal of a Geocities web page. This would be a real opportunity to create something that looks like nothing else in the market, so don't just look at other software's eye candy and copy it. Bring in a designer who will give MC a unique look all it's own.
For the iTunes ex-pat, and I fall into this category (although I dabble in the audiophile category), MC needs to be easier to learn. It doesn't have to be like iTunes, actually the opposite of that; I want the depth the program offers. But I won't be the first person to say that it needs a proper manual. Figuring out a complex piece of software from a Wiki and a Forum is a terrible way to learn and find answers. I don't know how many hours I have wasted looking for answers to problems that should have been readily outlined. It could also use a little more hand-holding. I know, the idea of that probably makes the programming team cringe. But build in a more guided experience for some of the better features of the program and users will respond to that. If I want to set up a multi-room system like a Sonos, the program should be able to guide me through the steps. Likewise for room correction, setting up with my soundcard/DAC for the best possible output, etc.
For the casual listener, like my wife, who just wants to quickly play something now and anything else is an obstacle, MC needs to tier the level of complexity in the program. Partly a design issue, partly a functional issue, but there is simply too much up front. The nicely designed JRemote does a lot to alleviate these problems, but something really needs to be done to the main application. Find a way to create an interface that starts simple and expands options and features out as requested.
Oh, and streaming services, blah blah blah. I know they are hard, but you really need to incorporate them in a nice seamless way. It's the future, and your program will never fare well against the competition without them. Spotify, Amazon, Tidal need easy access from the UI, not via an awkward browser interface.
All in all JRiver has a great engine in an ugly body. It feels like software designed by engineers, for engineers. To really compete with the other options in the market (and if you compare features alone I don't think JRiver really has any real competition here) it needs to bring it's UI/UX up to a modern standard and give it some sex appeal, and spend the $$ to put together a proper manual that remains up to date and doesn't rely on it's users to write. Hire a professional interface designer. Tier the depth of the program so users can dig in as they become more experienced, rather than blasting them with every feature up front. There is no reason MC cannot appeal to all of the demographics above, but it will take some dedicated money and effort.