It would be great if theme/plugin developers can use the syntax of the already familiar meta_query used from WP_Query for retrieving comments + ordering by meta values is also something that would be great to introduce.
Right now the only way I can think of to achieve this is by writing a custom SQL query, which is first not suitable for all developers(not everybody knows MySQL :) ) and besides using custom queries is recommended only for end cases, where you can't use any other built-in function(because the structure of the DB can change, etc).
Basically if those features are implemented, developers will have greater flexibility about comments - the first things that come to mind are rating system(I know there should be a plugin out there for that already), separating comments by some parameter(let's say age/sex/country/etc, if there is some kind of sociological study) and many more.
I'm pretty certain that this should not be really complicated - as far as I saw the WP_Meta_Query is constructed in such manner, that it should already support comment meta queries :)
