Custom unit measures

Can I select an edge and say … ok, this is 1 Blender Unit? It would be awsome do to that.

Perhaps in the future, but not right now.
If you do some math, you can scale the entire model up so that said edge is 1 BU long.

Why is it so hard to implement … ?

It’s probably trivial to implement, but it is not something important at this point. Maybe in the future. Besides, Blender ain’t CAD.

i don’t know how would you do that in CAD even… i use AutoCAD and i normally would start from a specific point and add the next point more carefully, by, say, moving the cursor along the x axis and typing the distance by hand…

it’s not blender’s fault… it’s your technique…

It is pretty fast to adjust an action accurately enough in edit mode, when “edge length” is enabled in “Mesh Tools More” tab. Shift and/or CTRL helps to snap the action.

Of course it could be done quite easily, but what good would it be? A unit, by definition, is a standard measure of some quantity. You’d get little more than confusion (if anything) if it was “customizable”.
Anyway, when you begin your model, you can simply “set” your unit virtually, for example for a house 1 BU = 1 meter, for a computer 1 BU = 1 centimeter, etc.