Are we talking about all feature gaps that might affect home theater use? There are a few others too, although some of them may be harder (or impossible) to fix:
5) Remote control and media keys
On Windows MCE remote controls and keyboard media keys work as expected in JRiver. Using them on Linux is challenging and requires significant work on the user's part.
There are two separate remote/media key issues. The first issue is that some media key presses don't seem to be implemented or work correctly. For example, using a standard MCE remote that worked perfectly on Windows, I have never managed to get the remote control number keys working in theater view on Linux the way they do on Windows. This is the case even using the exact same Linux environment that the devs use for testing (XFCE4 on Debian). Theater view on Linux just doesn't seem to respond to the number keys on the remote control at all even though they're sending the correct keypresses. To be clear, typing numbers on the keyboard works fine, but the remote control number keys do not work even though they're sending the same keycodes! Check out this thread for a recent(ish) example of another user struggling with remote control configuration seeing a similar issue:
https://yabb.jriver.com/interact/index.php?topic=136975.0.
Second, there's a separate issue in that many common Linux desktop environments (DEs) (like Gnome, KDE, and I think Cinnamon too) consume some of the media keypresses and expect programs to use the MPRIS API to get them, but JRiver does not use MPRIS so using any remote or media key that the DEs "manage" requires a user workaround. So for many Linux users the most common remote or media keys (
play/pause, stop, volume control) will fail out of the box, and require some fairly laborious workarounds to get them working (remapping keys using custom JRiver configuration, and then remapping the remote to hit those new keys).
To summarize, if you spin up JRiver on a default install of Debian (which uses the Gnome desktop environment as a default), Ubuntu (uses a modified Gnome as default), or Fedora (also Gnome by default), the majority of the remote control keys just won't work with no feedback as to why they're not working, and only some of those keys can be made to work via workarounds on the user side.
6) Keeping the Screen on During Playback
MC does not successfully inhibit the display from turning off on Wayland desktops when audio or video are playing, even when the setting to do so is enabled. That means you have to wiggle the mouse every fifteen minutes or so to keep the screen on or turn off the OS level power management and manually handle or script your own power management (which is what I ultimately did on my HTPC boxes).
Because most Linux distros now default to a Wayland desktop (i.e. Debian, Ubuntu, and Fedora default to Gnome on Wayland), I think that will be a pain point for many home theater users. I have tested this on Gnome and sway, but because I think MC relies on xset to keep the screen on, I think this would be an issue on any Wayland desktop.
7) DVD or Blu Ray menus
This is a subset of your 2), but an important one. For several years MC for windows could play folder formats, but didn't support menus, which is limiting if you're using a remote control/10 foot interface. Now I think MC for Windows supports Blu ray menus, which would ideally also be plumbed in for Linux.
8 ) Integrated Browser/Netflix
The windows version now has integrated netflix handling (and maybe other services too?), which is absent on Linux, but that may be unresolvable because of the DRM requirements and the embedded browsers available.
9) Television Support
The Windows version supports a wider variety of television tuner hardware than the Linux version; even some Linux tuners that have drivers and are supported on Linux are not supported by MC, I think because there's no unified API for MC to target.
10) Automatic display mode setting
On Windows JRiver can automatically change the refresh rate (among other things) of your display to match the content. There's no equivalent functionality on Linux (the Video dialog options are there, but don't actually provide any choices other than "desktop settings").
There are some other gaps, but they're either minor or wouldn't affect a cinema type setup. Credit to the JRiver team for closing a lot of other longstanding gaps over the years (i.e. TV support is much improved and CD playback and ripping now works).