I think there's a big difference between not liking something and thinking aspects of it could do with improvement.
Obviously it's always nice if someone says they like your tune, especially if you think they mean it, but I think most people are looking for constructive criticism, which is amazingly hard to find, especially from friends.
Useful things you can comment on about someone's tune, especially if you do production yourself:
- Programming
- Mix (especially this, as having people listen on other equipment can be very useful feedback)
- Sounds (drum sounds, synths, vocals, fx)
- Arrangement (this is something I personally suck at so I always like to hear suggestions on my arrangements
Really "that's great" and "that sucks" are useless information. If you can pick out something specific it's going to be much more useful to people. Whether you subjectively like a track overall is neither here nor there, because people's tastes are so different.