There are a lot of MythTV settings. Page after page of preferences. In many ways, this is bad design; a refusal by the authors to make decisions about correct and incorrect behavior. But that's the situation, and here are a few settings where I think the value should change.
Utilities/Setup.Setup.General.Audio.Enable AC3 to SPDIF passthrough
If you have your Mac hooked up to a receiver via an optical cable, remember to check this or you will throw away your surround sound. Unfortunately, it doesn't auto-detect the absence of such a cable and fall back to non-passthrough, so you'll end up unchecking this when you move your Mac.
If you are using a widescreen Mac, or attaching your Mac to an HDTV, you should try MythCenter-wide as the theme. Non-wide screen themes tend to get their lower buttons cut off in the more complicated preference panels.
The Mac themed buttons are unusable (transparent blue on blue). Use Windows style.
Utilities/Setup.Setup.TV.Playback.General Playback.Deinterlace playback
Unless you have a Mac which can just barely display 1080i content, or your monitor will deinterlace content for you, you should turn this setting on.
This is something you should investigate for yourself. Run the Activity Monitor application in your /Applications/Utilities folder to determine the performance hit of the various deinterlace methods, and test them for effectiveness. In particular, the BOB method doesn't work well for me, while the Kernel method works but uses more of my CPU. I will try using the One field method.
Utilities/Setup.Setup.TV.Playback.On Screen Display.OSD Theme
The Gray-OSD theme seems the the most tasteful of the bunch.
Utilities/Setup.Setup.TV.Playback.On Screen Display.Always use Browse mode when changing channels
If your channels take as long to change as mine, you'll prefer to see what's on before committing to changing the channel.
Utilities/Setup.Setup.TV.Playback.Mac OS X Video Settings 2/2.Video in the dock
I think this feature is pretty useless and a waste of power.