- if you use core profile, modern GL, there aren't a lot of quirks. It's a fairly nice API overall.
- if you target reasonably modern drivers, you won't typically find many inconsistencies and issues. If you do something unusual (like the latest idtech with megatexture) you might run into driver issues -- but that's just how things are, it's not the fault of the API. New functionality needs testing, and GPUs are still constantly evolving.
- extensions are not useless. There are a huge amount of absolutely fantastic extensions. This is, IMO, the main strength of GL over D3D (and other APIs), which does not have a mechanism like this. If you know the hardware supports a certain functionality, there will most certainly be a GL extension to exploit that functionality, even if you can't with D3D.
- in particular, KHR_debug is the best thing since sliced bread. It allows you to have the GPU driver diagnose your program for you, and give you hints such as:
- things you do wrong
- things you should do differently to increase performance
- general debug information
- information on memory usage and where your buffers are stored
I've been working on something recently using modern OpenGL though, and i've found it a lot more manageable. Because there's no longer a fixed pipeline -- you have to write your own shaders to be able to do anything 3D graphics related, the pltform is vastly simpler, to the point of being relatively easy to understand, if still a little complex to use.
The API is still quite crusty; there's little in the way of abstraction, and you often have to make quite a few calls to do simple stuff, but it's conceptually a lot cleaner.
* OpenGL core profile is quite clean and lean. Both apple and intel have already decided not to implement OpenGL compatibility (where all the cruft sits). There are still a few things that can and should be cleaned up, but it's pretty great already.
* Adding an additional API means there will be YET another thing vendors have to support and will be able to screw up, in addition to the already existing plethora of APIs: GL, D3D, DDX/GDI, OpenCL, VDPAU [or something like it], CUDA [or something like it], OpenMAX, Mantle. OpenVG, ... -- nobody is going to drop OpenGL or D3D for something like mantle, because that'd instantly lose you all your customers. Not even AMD.
* AMDs mantle (which is, as far as we know, just a specific thing to their cards and not even portable to any other cards) is not an attempt to make things more stable, but an attempt by AMD to gain more control over the market (something they are in a good position to attempt right now, as they have control of the console market) and to push into the direction of a low-level interface that is strongly tied to their hardware model. Mantle as an API is (as far as we know) even lower-level than GL and D3D, and hence not very suited for most programmers and applications -- there are a lot more things you can do wrong with it.
Don't believe all the FUD you read on hackernews. The main issue with OpenGL right now is that some vendors are doing a shitty job at it, which is mainly due to the heavy influence microsoft had on the gaming industry for a long time. GL is now making a major comeback due to linux, apple and mobile platforms gaining popularity, which will help rectify these issues over time.