What is so bad about GL_QUADS?

I hear that GL_QUADS are going to be removed in the OpenGL versions > 3.0, why is that? Will my old programs not work in the future then? I have benchmarked, and GL_TRIANGLES or GL_QUADS have no difference in render speed (might even be that GL_QUADS is faster). So whats the point?

The point is that your GPU renders triangles, not quads. And it is pretty much trivial to construct a rectangle from two triangles, so the API doesn't really need to be burdened with the ability to render quads natively. OpenGL is going through a major trimming process, cutting a lot of functionality that made sense 15 years ago, but no longer match how the GPU works, or how the GPU is ever going to work. The fixed function pipeline is gone from the latest versions too, I believe, because, once again, it's no longer necessary, and it no longer matches how the GPU works (programmable shaders).

The point is that the smaller and tighter the OpenGL API can be made, the easier it is for vendors to write robust, high-performance drivers, and the easier it is to learn to use the API correctly and efficiently.

A few years ago, practically anything in OpenGL could be done in 3-5 different ways, which put a lot of burden on the developer to figure out which implementation is the right one if you want optimal performance.

So they're trying to streamline the API.

People have already answered quite well on your question. On top of their answer, one of the reason that GL_QUADS being deprecated is because of quads's undefined nature.

For example try to model a 2d square with points (0,0,0), (1,0,0), (1,1,1), (0,1,0). This is flat quad with one corner dragged up. It is impossible to draw a NORMAL flat square in such way. Depending on drivers, it will be split to 2 triangles either one or another way - which we can't control. Such a model MUST be modeled with two triangles. - All three points of a triangle always lies on a same plane.

It isn't "going" to be anything. As with a lot of other functionality, GL_QUADS was deprecated in version 3.0 and removed in version 3.1. Obviously this is all irrelevant if you create a compatibility context.

Any answer that anyone might give for the reason for deprecating them would be sheer speculation.