In 2010, I picked up my first camera, an IstDS. Not a breathtaking camera by any standards, but it was what got me into photography and made eventually switch my major to photography. Since then, ive gone from the K10 to the K20. The nikon D3200 (an entry level camera) has much better image quality, better maximum iso, not to mention noise performance than then my K20 (considered to be a midrange camera) Just a little while back, I was at the lake trying to take some shots of a group of birds. I was trying to catch them in mid-flight/mid take-off but the autofocus on the camera was miles too slow. By the time it had locked on, my subjects were long gone. Pentax seems like it takes a lot of adjustments and tweaking to get the results you want, whereas nikon just helps you focus on getting the shot you want.
User friendliness seems like its always been an issue with pentax cameras. A good example of this is the "custom image" and the image filters which are in completely different places. I've come a long way as a photographer since picking up my first camera and i feel like i need something that just helps me focus on my creative process rather than having to wade through difficult menus. I messed around with my friends nikon a few times in class and i was amazed at how intuitive and easy it was to use.
I think this quote from The Verge sums up pentax's issues with user interface pretty well.
Quote: The other problem is a more systemic one, of which Pentax is but one of a number of guilty parties: it just takes too long to find and access settings. For instance, changing the LCD color scheme takes 16 taps, and you'll have to read about 100 menu items before you figure out that LCD Display is in the first page of the section with a wrench icon. How the hell are you supposed to figure that out? A few cameras, like Sony's NEX lineup, are starting to come up with smarter and simpler ways to organize and surface settings and options — Pentax needs to get on board.
I feel like the only significant thing I would really be giving up is the weather sealing. Given that I do a lot of outdoor model photography, I dont think most of my models would want to shoot when it was raining and to be honest, neither would I. Ironically, im watching a review for the Nikon 7100 and the reviewer is walking around in the rain, umbrella in one hand and camera in the other.