The purpose of cabal lint command would be to point out potentially bogus things in the users .cabal file. It could additionally check for violations to hackage's policy. The idea is that, while cabal checks for serious problems automatically and gives an error, there may be other things it could warn about (if they are correlated with mistakes but not necessarily wrong).
Here are a few things off the top of my head to get ideas going:
Basically, anything that is questionable but not obviously an error could have a cabal lint warning.
Instead of adding a new command, isn't it better to just extend cabal check?
actually, a cabal lint would be useful for a different (but related role): checking the users ~/.cabal/config settings!
Currently theres no way to give nice user feedback about potential issues with having an old ~/.cabal/config, and this will likely hit a few users who try to use cabal 1.18 and ghc 7.8 with the old no enabled shared flags that might be lying around in a legacy config
If there's a potential issue with the user's ~/.cabal/config, that needs to be a warning wherever it may be an issue. But, I'm not aware of any issues that did arise during the Cabal 1.18/GHC 7.8 transition.
Tagging this documentation for the outstanding issue that cabal check is not properly documented.