With all the rage of the paint and primer in 1, I just want to talk about this briefly.
I have no issues with a lot of the paint and primer in 1 products. The ones that I use are really good products, but there are times when I know a specific primer is needed. We will not talk about primers right now. That's a whole different ballgame.
I keep my ears open when I'm around home improvement stores, paint stores, and even my estimates (of course) and I am worried that people are assuming that paint and primer in 1 means that only one coat is needed. In my opinion, 2 coats on anything that has been previously painted will give the best results but since cost is often associated with painting because it involves a lot of tedious man hours I often give prices for 1 coat on the following previously painted surfaces:
Flat Ceilings - Because dead flat paint covers very well and does not flash. 1 caveat is that it should be the same color or very close. Repairs can be spot primed before painting. I recommend Benjamin Moore Ceiling Paint for 1 coat.
Doors & Trim - Since these areas can usually be painted "wet on wet" without overlapping, the finish is uniform. As far as coverage, again, color should the same or very, very close to the existing and I high quality enamel used. I recommend Sherwin-Williams ProClassic for 1 coat.
I don't feel walls should ever receive just 1 coat no matter what the finish is. Sheen finishes, such as Eggshell or Eg-shel, Low Sheen, Satin, Pearl, Low Lustre need 2 coats for not only full coverage but sheen uniformity. For large surface areas, overlapping will occur when some paint dries and fresh paint is applied over it. This will create sheen differences and is often noticed when looking at an angle.
If you absolutely must put 1 coat on a wall, flat paint is your best option. This is a common practice in rental properties.