OK, took long enough to find some time, but here we go:

• Most customers are not trying to cheat you.
• Complaining is stressful, particularly for the conflict-averse. By the time a customer calls/emails, you're already behind and have work to do to repair things; a significant proportion will not complain and will just go elsewhere.
• If you're going to quote a rule at a customer, make sure you understand it and have all the details correct. It's spectacularly irritating to have to explain your own Ts&Cs to you.
• If you're going to quote the same rule at a customer again, escalate the call. They say doing the same thing and expecting a different result is a definition of madness...
• If you don't understand where a number comes from, admit that and find out, don't just make something up.
• If the customer is obviously dissatisfied with the service provided, or presents you with an ultimatum, escalate the call.
• Never, ever, hang up on a customer or stop responding to their emails. If you really don't want to deal with them, escalate - the feeling is probably mutual by that point.
• Mishandling a complaint is often far more serious than the original issue. If you're not part of the solution, you're part of the problem.
• If you're going to offer differential quality of service (i.e. Diamond Desk) for some customers then a) make sure they have access to it from the moment they're eligible (an email, perhaps? or the contact details available on the website when logged in?), b) make sure that the service actually is better, and c) make sure that it's coordinated with standard customer support.
• The customer has no interest in how your business is structured, how your systems work, or who is going to be dealing with a problem behind the scenes, only what he/she will see and hear as a result.
• Setting low 'contractual' targets ("allow up to five business days after check-out from a North American hotel, and approximately two weeks from a hotel outside of North America"; 6 weeks of processing time for bonus-offer certificates to be sent out), exceeding them in normal circumstances (stays post in 48-72 hours; Q3FNs emailed within a few hours of the final qualifying stay posting), but then relying on the letter of the law to buy time when something goes wrong is not acceptable. It just means that the customer has lots of time to get nice and angry before they're allowed to complain.
• The customer is always allowed to complain.
• Your rules are your rules. We may be forced to tick a box saying we agree to them, and that might even be legally binding rather than a textbook example of an Unfair Contract, but it doesn't actually mean that we do agree to them - just that it's impossible to book a stay without ticking the box. Common sense is far more important than the wording of the Ts&Cs.

And a couple of more general ones that aren't directly about customer service:

• Poorly thought through promotions that result in members getting significantly different benefits to others of the same nominal status are divisive and as likely to lose business as to gain it - rewarding people who don't give you their custom, in preference to those who do, is the antithesis of loyalty. Calling it something like "Diamond Retention" doesn't alter the fact that you're rewarding disloyalty.
• Loyalty doesn't have any marginal benefit after 28 stays/60 nights/$10k - it would actually be better for me to "max out" several schemes and therefore get a much greater selection of hotels who will treat me as a top customer. I don't claim to have the answer, but a bit of thought suggests that maybe making the tier points bonus (22+#stays)% would make each stay in a year more valuable (and similarly for spend/nights).