Now the door is tight and opens and closes easily.

You could easily go upside down with a few extra vacancies, difficult evictions, or major repairs.3) you cannot be afraid to walk up to your own property and knock on the door to get to the bottom of an issue

I know what neighborhoods were up and coming, and can get everywhere very easily.

How I would determine it: if the unit tends to be difficult to rent; if it attracts the kind of renters I try to avoid; if I could easily replace it at a lower price and make a significant profit from selling it; if I had too many units or was tired of dealing with that particular unit (high maintenance, for example). 

. ;) I prefer solid hardwood mostly because it can easily be refinished, but i'm seeing a lot of benefit from going with the engineered products too.

Both are good options that can easily be held for years to come.

This is a reasonable number that one can easily acquire in 5-10 years if they put just even some of their pay check to the side each month.

If I added a single parking spot to each unit in my duplex, I could easily charge an additional $300+ a month in rent (per unit).  

Seems there is a lot of grey and the rules aren't exactly laid out to easily be consumed.  

The IRS does not state that a taxpayer must hold for a certain timeframe but with that being said if you hold the property for 24 months, you straddle three tax years and should easily be able to prove your intent, under audit.