I have also just received my K3, and am enjoying it. The last SLR I had was a Canon AE1 which I would gladly have stuck with Canon had the lenses I had on that body been forward compatible with new Canon equipment. But they weren't which allowed me to look at things as completely starting all over (although I had a series of very early adopter first generation of Digital cameras from Minolta which still work but just don't have the lens capabilities).
I'd always look that the situation you have (or are intending to be in) to have an idea of what you are after. I see people are recording a Sigma 70-200 f2.8, but I looked for and am quite pleased with the Tameron "equivalent" which to me makes more sense for a Pentax body since I didn't feel like paying for an extra image stabilizer in the lens itself.
I do some photography with high school cheerleaders and it's indoor competitive season, and the lighting tends to be florescent, so to me while I don't have to worry about weather sealing indoors, poor lighting is something I am still trying to work around. I am amazed at how different the faster lens is in terms of the amount of light it gathers compared to the smaller 18-135 that I got with the camera. I like that lens, but compared to the faster bigger lens, I feel like I'm able to get much better shots just because at the same f stop of the bigger lens I think I'm pushing a couple of faster speeds on the shutter speed.
The bottom line to me is lens speed for this, but I don't want to crop everything from a prime, so a fast zoom still makes sense for me.
Here's an example of what I can capture, and remember they're moving pretty fast when they do this stuff
(F3.5, 1/400, 70mm):
But what it really comes down for me (at least) is that it you should have in mind what your after, then pick the tool. Otherwise if all you have is a hammer that everything will look like a nail. Over time, the lenses that make the most sense will become apparent. I look at it as a whole system that will be built up over time.
(but then again, the "best camera in the world" for you is the one you have with you at that moment)