I'm writing about rect class realization I believe rect constructor argument names are confusing and should be changed.
So this how current rect constructor looks in C#:

CSRAW class Rec(float left, float top, float width, float height)

And this how it should look according me:

CSRAW class Rec(float x, float y, float width, float height)

Justification:

For example I want to create new Rect.

1. Rect rect = new Rect(0 (left), 128 (top), 128 (width), 128(height))
2. So I should expect two possible results (if width goes down or it goes up)

a) Rect - left = 0, right = 128, top = 256, bottom = 128.
b) Rect - left = 0, right = 128, top = 128, bottom = 0.

Of course the right answer is A. However according the documentation picture (http://docs.unity3d.com/ScriptReference/Rect.html) the right answer is B. I know that this argument naming is came from OpenGl API orientation system, but usually user doesn't know this and it can be extremely confusing.

I also had some discussion with developers, so this is their thoughts:

@Mads Nyholm <mads@unity3d.com>
"I think we should simply change it to: Rect (float x, float y, float width, float height)

Internally is sets xMin, yMin, width, height.

The use of the properties left, right, top and bottom has been made obsolete a long time ago."

@richardf@unity3d.com
"I’d prefer (float xMin, float yMin, float width, float height) over that, but sure."

@Aras Pranckevicius <aras@unity3d.com>
"Yeah, the fact that we have two different coordiante spaces is confusing, isn't it.

Many things in Unity use "OpenGL like" coordinate space. For example, texture.SetPixel with (0,0) means "bottom left"; same with some coordinates like in Camera.ViewportToWorldPoint, where y=0 is "bottom", or with Input mouse coordinates, or camera viewport settings.

However! IMGUI (i.e. all the OnGUI stuff), and I think the 4.6 UI uses y=0 as "top".

To add to the confusion, the super-legacy GUITexture and GUIText components of course use y=0 at "bottom".

Fun :)

...I agree, renaming the arguments to be (x,y,widht,height) would make it less confusing. And also perhaps stating which parts of Unity use which coordinate conventions somewhere (see above) too."