For cash flow it can't be beat, but for appreciation it is not so good and it is easy for out-of-state people to overpay and not know it.

Learn the rules in your area.

.-- Boulder County just simplified their rules to make it easier, in my opinion, to do STRs. -- Grand County, where Grand Lake is (also Winter Park), allows for vacation rentals pretty much unabated. -- Routt County, where Steamboat Springs is, has totally banned them.-- A little further south from you, the counties and cities around Denver are a little tougher on short-term rentals. -- Clear Creek County and Gilpin County both have caps on the number of Airbnbs and have met those caps. -- Jefferson County technically allows them but a county board has discretion over renewals, and I know of people being denied for what seem like arbitrary reasons.-- A little further from you, Park County is a great option.

Also community rules, rental restrictions, pet restrictions, occupancy restrictions - anything that could interfere with your operations. 

It was easy to keep a high occupancy rate with the location being close to 2 universities.

Your city should have enforcement rules that require him to maintain his property in a manner that prevents damage to your property.If the city doesn't help, then you can try paying for it yourself and sending him the bill.

Is there a generic rule anyone follows that tends to work? 

Two completely unrelated exceptions to the general rule.2.

Are there any rules preventing me from asking about their pet?

They chose to refinance at 70 LTV to keep their cashflow higher, which meant that their new loan was 164K and they took out their initial downpayment + close to 30K additional... leaving no money in the deal with a deal that still fit the 1% rule and cash flowed.