1

Any spray gun that doesn't have a hook on top.  Come on folks there are some big manufacturers that STILL don't put a hook on top of their latest guns.  While you're cleaning the gun you've got to lay it down on a dirty bench until you put the cup back on in order to hang it.  The guns may work great but the lack of a hook is VERY inconvenient for daily gun use.

2

 Spray gun inlet pressure gauges that go from 0 to 160 PSI but the gun needs to be set at 29 (or whatever) PSI for proper performance.  This means that the area of the gauge is about 1/4" for every 10 PSI so it can be quite difficult to get the pressure right on the correct setting.  If the gauge only went to 50 or 60 PSI the setting would me much easier. This is a problem with MOST inlet gauges.

3

Square gallon paint cans that are labeled so that you can't read them when they are on the shelf. Cans are put on the shelf like books with the side of the can showing, not the face.  Some manufacturer's only put the readable part of the label on the face which means you either need to pull the can off the shelf to see what's in it or you need to use a magic marker and label it yourself.  This is a problem with some inexpensive and expensive products.

4

Automobile paint codes that are difficult to find and/or decipher.  Can't the oldest car manufacturers follow the ways of the newer manufacturers and put the color code on a label where it can be readily seen and used.  The US and European manufacturers are the worst for this and sometimes, even when you know where to  look, the code is buried in a bunch of other numbers and letters and takes additional information to "decipher".