"anyone can do that?"

Yes. Anyone can do that. Provided that this "anyone" puts in the time and effort to master the tools, learns to master composition and lighting. Which takes a LOT of time - a lifetime.

Poser and DAZ|Studio offer some shortcuts. You don't have to model human figures from scratch - which is very, very difficult and takes months, if not years, to master - but you STILL have to put in a lot of effort to get a good composition and light the scene well.

Some artists use Poser as it was originally intended - a quick render with basic lighting, then bring the resunt into a 2D program and postwork the hell out of it. Which can lead to excellent results, if the artist put in the time and effort to master Photoshop/PSP/Gimp/Painter/whatever.
Others spend hours, sometimes days, to fiddle with magnets and deformers, lighting and materials, until Poser / DAZ|Studio delivers a rehder that is "finished" by itself and doesn't really need postwork to bring it up to quality.

As for the quality of images in the 'rosity galleries: variable. From very basic beginner to sublime works of art. 

Nothing new here. It's all been said before.

About the questions the original poster asked:
1): Depends on the relation between the skill level of the artist and the artwork itself. A very skilled artist may deliver an image which is good, but "cookie cutter" for his/her standard - easy stuff, level 1.
And a beginner may put all he/she has in a work of art, driving himself to a higher level, producing an image that, measured with "objective" standards (do those even exist when it comes to art?) is of mediocre quality, but which is the best that he can do. Dificulty level 10+ - for this artist.
Conclusion: this question is not relevant.

2): Am I skilled? I'm not an absolute beginner anymore, but neither do I know and use my tools to the limits of their capacity. I'd say I'm at an intermediate level.

3): I've been at this 3D stuff for 4 years, and I just keep on learning. I'm convinced that I'll keep learning for the rest of my lifetime. I don't know it all, and I never will, but every time I work on a scene I learn something new.

Comments and criticism on 'rosity: most comments are ego-boosters. It's always nice to read that someone liked your work, but it does not help to improve your skills. I certainly don't object to comments like "fantastic work! Love it" but I'd rather see comments like "Look at the left feet of your character, a stronger shadow there would make sure his foot looks planted solidly on the ground" 
That is constructive criticism, of which there is far too little.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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