Originally posted by aliquis Birds don't interest me. I'm vegan so killing animals doesn't either. But I guess you could shoot them with a camera. But it seems like something which would take a long time. I would most likely rather shoot people (with and without camera ;D), wide angle on buildings and cities and shit and eventually wanted to have a nice macro, where Canons 65mm 1-5x is the most awesome thing there is I guess. For nature shots which are not birds
Birds are boring
It all depends on what you want to shoot right? And I agree, birds are beautiful, but they are boring. I shoot them occasionally, but mostly at the zoo
To me, it is a good to begin with an idea as to what kind of photography you're interested in. If you're shooting mostly people and buildings, long glass isn't really necessary, but a wide to normal telephoto would be important. This could be anywere from 12mm-135mm.
I think any of the cameras you mention would do superbly in terms of output. And all brands offer lenses to suit that range. Soooo, to me, it would then come down to glass and flash systems. And then from there, it would be zooms vs. primes and wireless flash systems.
I think Pentax zooms are the best bang for the buck when you compare them to equivalents from Canon or Nikon (i.e. anything that's f/2.8). Pentax's combo of DA* 16-50mm f/2.8, 50-135mm f/2.8, and 60-250mm f/4 are much more affordable then say, Nikon's 17-35 f/2.8, 24-75 f/2.8, and 70-200 f/2.8. I'm not familiar with Canon's offerings so I can't say anything there. For ultrawides, Canon and Nikon both have the Tokina 11-16mm f/2.8, which is cheaper than Pentax's 12-24mm f/4. Of course, there are other offerings from Sigma and Tamron made for all brands in the ultra-wide, but I can't think of any with constant apertures.
Now where I think Canon and Nikon have the upper hand, is in readily available cheap primes and a better flash system support. In Nikon land, the 35mm 1.8G, 50mm 1.4D, and 85mm 1.8D to name some. I'm not familiar with Canon's offerings, but these Nikkor lenses are affordable (relatively speaking), readily available, and even cheaper used. And they are a good range for "people pictures".
With flash, as I venture more into the world of off-camera flash, I see lacking support for Pentax in terms of wireless radio triggers. Although there are Pocketwizards, radiopoppers, and others available, none can transmit TTL info for Pentax. Of course you can be a manual strobist, but why not take advantage of technology? Anyhow, Pocketwizard and radiopoppers support TTL transmission for Nikon and Canon, but not for Pentax.
So, there are some points to consider, but you can certainly make great photos with any modern camera. Or old ones too