I switched from P&S to DSLR for a few reasons. I wanted a faster camera with more frames per second. I wanted more megapixels, because more is always better, right? I wanted better photos, and DSLRs are capable of out performing P&S in image quality. Basically, my first few P&S cameras got me hooked on wildlife photography and then I wanted bigger, better, faster, etc. After doing some research, I bought my first DSLR and a 70-300mm lens. My image quality instantly jumped to a whole new level and I was quite happy with my purchase. After a while I started to get frustrated though, my lens didn't have the reach that I really needed to get me close enough to most of the birds I shot, so I invested in a 50-500mm lens. The longer lens quickly took my photography to another whole new level and I was quite happy with my purchase. After a while the camera manufacturer came out with a new model that had some features that I thought I needed so I bought a second DSLR and I was happy with my purchase. After a while I started to notice that my images had a lot of noise/grain/speckling in them and soon realized that the 2x crop factor(smaller sensor) in my DSLR was contributing to my noise issues, so I changed manufactures and bought a whole new set up. I bought a camera with a 1.6 crop factor and a 400mm prime lens(no zoom) since most of my shots were always at maximum zoom and seldom did I need a shorter focal length. My image quality jumped to a whole new level, again, and yet again I was happy with my purchase. After a while I bought a 2x extender for my lens and although I seldom used it, I was quite happy with it's performance when it did get used but that 2x 400mm combination was limited to being mounted on the tripod so it got used less and less. The 2x extender also wouldn't auto focus on my camera so using it had the added frustration of trying to manually focus my shots, sometimes I got it right, sometimes I missed. A newer version of my camera came on the market that allowed for centre point auto focus with a 1.4 extender so I bought the new version camera and the 1.4x extender and was quiet happy with my purchase. After a while I decided that I wanted the newest version of my camera manufacturer's 100-400mm lens, which not only had image stabilization that my 400mm prime lens was lacking, the 100-400 zoom lens was also claimed to be as sharp as the 400mm prime lens, which I can't dispute, it's a great piece of glass. I am now noticing that my images are often noisier than I'd like, especially in low light conditions such as early morning or late evening. I push my ISO up higher than I should so that I can get fast enough shutter speeds in those situations, and the results often show noise that I'd rather not be there. In order to get rid of the noise from high ISO settings I will need to invest in a full frame camera with a bigger sensor that produces little to no noise at the same or hopefully even higher ISO settings than I currently limit myself to. A full frame camera would also allow for multi-point focus options with either extender. Yes, a full frame camera is now on my wish list. So that's my experience in purchasing camera gear, I learned a lot of lessons about what I needed/wanted the hard/expensive way. Knowing what I know today, I would have saved myself a bundle of money and bought a full frame camera, a third party 150-600mm lens, and then round out my setup with a 70-200 F/2.8, which is a workhorse of a lens, and a shorter zoom lens for the fewer landscape shots I take.
All the while I was making mistakes about my camera gear purchases, I was learning more and more about photography. I started off in auto mode with my P&S cameras and tried the various semi-auto settings like Program Mode, Sports Mode, Aperture Priority Mode, Shutter Priority Mode where I could control things a bit but the camera still took care of getting the exposure right for me. I found that I could use Aperture priority to set my lens to it's fastest/largest aperture, set the ISO to a fixed setting of my choosing, and the camera would come up with the right shutter speed for a proper exposure, most of the time. When I got my first DSLR I started playing around with exposure compensation, which is used in the semi-auto modes to adjust the brightness of the shot. I was getting too many silhouette images when I was photographing birds with a bright sky behind them so I learned that I needed to set my exposure compensation to +1, +2, or sometimes +3 in order to get the bird exposed properly in those situations. I had a hard time remembering to reset my exposure compensation after shooting birds up high with the sky/clouds behind them when it came time to take a shot of a bird with a darker background behind it. I think it was about that time I decided to try full manual exposure. After all, I was setting my own aperture, setting my own ISO, and fine tuning my own exposure with the exposure compensation feature in the semi-auto modes so why not try full manual exposure. Yep, I botched a bunch of my first attempts at manual exposure but it didn't take long to learn from my mistakes and before I knew it, I was controlling the exposure and getting satisfying results. I found it so much easier to control the exposure in manual mode than in any of the auto modes, including the ones that allow exposure compensation. If a bird was in the sun, it wouldn't matter if it had a bright sky behind it or shaded bush behind it, the exposure usually didn't need to be adjusted. If the bird moved out of the sun and into the shade, I simply adjusted one setting on the camera and I was good to go. As I learned more and more, through practice, reading online sources, and the hard lesson of learning from my mistakes, I kept getting better and better shots in manual mode and would never go back to any of the auto modes. There are three basic adjustments in manual mode, aperture, shutter speed, and ISO, which are easy to adjust on most DSLRs. Learning which adjustments to make and when you need to make them is something that comes with practice/experience but once you set your exposure in manual, it doesn't need to be adjusted often, or adjusted much. If you have already learned how and when to use exposure compensation in the semi-auto modes, you already know how to set the exposure manually, you're just not doing it in manual mode.
So, that's my experience in both purchasing and learning the controls of my cameras. Am I great photographer that you should take advise from, probably not. I consider myself to be a decent photographer with decent equipment that takes some decent photos but I admit I still have a lot to learn. If I was to offer advice on purchasing camera gear and learning how to use it, I would recommend getting a decent long lens, the longest quality lens you could afford. The better the glass is, the more likely it is that you'll like the results it produces, now and in the future. While I know that full frame cameras have better image quality than crop cameras do, I'm not sure if starting with a full frame camera is something that I'd recommend, I might suggest the idea but I wouldn't consider it a necessity. I would advise against cameras with the smaller sensors though, a 2x crop factor might give the illusion of a longer lens, but the smaller sensor simply can't capture the same detail that larger sensors can. Small sensors with 18MP means 18MP of small receptors(for lack of a better word), larger sensors with 18MP means 18MP of larger receptors. The larger the receptors, the higher the dynamic range, meaning more versatility in capturing the brightest and darkest elements without over exposing and/or under exposing the image, and less noise in the image too. As for what to learn first with a new DSLR, I'd recommend learning the cameras controls for ISO, shutter speed, and aperture, learn how to adjust them so that you can do it efficiently in the field, regardless of what mode you're using, manual or semi-auto. I highly recommend going straight to manual mode though, it's not as complicated as it may seem and it will teach you a lot quicker than any other mode on the camera. I also find it much more gratifying when I get the shot right with manual exposure, knowing that I was the one controlling the outcome of the photo and not letting the camera control it for me just brings a greater sense of satisfaction for me.
If something I've written is confusing, feel free to pick my brain. Just don't start asking what camera you should buy.