While you can't create limitations against their lease and legal rights they have for quiet enjoyment, things like keeping common areas free of clutter or routine maintenance items that are tenant responsibility can often be added to help keep the expectations on both parties clear.

This is more common that you might think.They do not take into consideration the costs borne by the investor associated with the transaction such as: closing costs, carrying costs, finance costs, exit costs, etc.They are trying to hit a homerun on every deal and not leaving enough "meat on the bone" for the next person. 

Its' not the rule but certainly common that filth grows with long term tenants.

I've been experiencing lenders tighten there underwriting, and a 1.3X DSCR has been somewhat common in my experience.

I would say a common error that people make is underestimating the cost to fix or repair something.

If this is becoming common in certain areas, do you think it might deter a significant percentage of potential guests?

This is incredibly common in Philadelphia's market if you don't invest in the correct neighborhoods.  

This is fairly common in college towns so I'd recommend verifying that before going off on a buying spree.

First, I would recommend calling around to apartments and other PM companies in your area to find out what is common. 

Most postal carriers have enough common sense to recognize which box is for unit A and which is for unit B.