A search brought up (only?) several threads and neither actually contained a good answer to the problem. I rate photos with a professional image management application (ACDSee Pro6) which embeds it's metadata into tags that Windows and other applications are able to read. I have confirmed that the embedding is complete and I can see the ratings in Win Explorer (and Win Live Gallery, Win Media Player, Adobe Bridge/Creative Suite apps, etc.) However, photo ratings are not imported into MC18. In MC, the "Rating" tag shows "? stars".
I suspect it's because ratings are stored in XMP tags and there's no official standard for the ratings tag. However, if Windows and other programs read the rating, then there's no reason why MC shouldn't be able to (rather, MC should read these).
Here's some specifics that should help track down the metadata fields in question:
I have an image that does display the rating in MC. That's because I used to manually create the tags in a commandline utility in a pretty onerous process. For this image, the ratings are contained in the following fields (derived from the Raw XMP data):
<rdf:Description rdf:about='' xmlns:xap='http://ns.adobe.com/xap/1.0/'>
<xap:ModifyDate>2011-05-05T18:36:05.34-6:00</xap:ModifyDate>
<xap:Rating>5</xap:Rating>
</rdf:Description>
For a second photo where the rating does not show correctly in MC (but does almost everywhere else) the rating is stored in a similar, but slightly different way:
</rdf:Description>
<rdf:Description rdf:about="" xmlns:xmp="http://ns.adobe.com/xap/1.0/">
<xmp:Rating>4</xmp:Rating>
<xmp:Label/>
</rdf:Description>
Hopefully, the devs can make the MC image rating import a little more universal so I can finally use it to serve photos over DLNA.