I don’t have much of a comment policy, but I do adhere to some general practices:
1. All spam gets deleted.
2. All comments in languages I personally do not know get deleted.
3. Comments that make no sense (and I mean really make no sense, e.g. rambling, off-topic lines from poetry) get deleted. If you wish for your whimsy to survive moderation, preface it with a sensible heads-up.
4. Comments from first-time contributors enter the moderation queue. Be patient.
5. Comments with two or more links enter the moderation queue. Again, be patient.