What can be done about a moderator who makes a practice of deleting comments or posts s/he apparently disagrees with? (Note that these are generally factual answers, not just expressions of opinion.) This has been the ongoing (years, IIRC) practice of one particular person, and is getting rather annoying.
For Comments - Give them a pat on the head and tell them "Good Job"
Comments are meant to be transient, and for clarifications and other such things that don't quite fit into an answer. They're not meant to be something left for the long run. Clearing out comments is one of the things moderators do a lot.
As for posts - it depends. After quite a bit of time on these sites, I've found that scope, for questions and the definition of a valid answer varies. The best person to talk to in this case is the specific moderator.
Assuming that there's a sense that posts along a certain theme, in scope are deleted, it might be worth bringing it to the community in a clear, compelling meta post. It might not work always - for example, Super User's policy on hackintosh questions wasn't changed, but it at least encourages discussion and visibility on the reasons for this. You might be convinced. You might not.
Broad meta posts like this arn't helpful IMO. They're primarily useful for hitting people over the head with inappropriately if folk fall for it. So, what can be done? Talk to them, make a specific case for these questions. Help set the policy by being constructive about the specific topic for posts, and why making a case for they shouldn't be deleted.