If you were early you did fantastic, if you were in the middle you did good (which is where your approach would get you in) I would rather learn all the indicators of an emerging market (not just the broad strokes like population increase, supply/demand, job growth etc.

I would be more concerned with small/emerging markets rather than those that have had vacation rentals since horse and buggy - like the Panhandle.   

Egress windows are requriered for a reason, to allow exit during an emergency, typically a fire, most fires start in the kitchen and most basement stairs are right off the kitchen (granted not all but most).

I include instructions for payments, reporting ordinary maintenance/problems/questions, and a second method for reaching me with emergencies.

I won't take them to the emergency room. 

Do you also provide a Welcome Basket with a list of Emergency Numbers and Other Local Information?

If none of the windows are openable, then it could be a higher liability in the long run due to lack of egress in an emergency situation, but ultimately that's an attorney/insurance provider question.

Be stuck with no emergency fund is no fun.

A few things are: fire separation distances (window/door opening location/sizes in relation to the property lines), easements (hopefully none of these garages were built on city easements), egress (adequate emergency escapes from bedrooms), utilities (are they separate meters and were they installed up to code?) 

I'm far from an expert in real estate but my observation is that the real estate industry is being disrupted and new business models are emerging.