This is the advice a friend of mine gives. His cameras cost thousands and he is a prize winning photographer. I am not a photographer, so have no idea of the merits of his advice, but take an interest in it because of a general interest in technology and because I have someone else to buy for. Here is a summary.
1) OS does not matter unless you are using RAW. Otherwise you are using a card reader, and any OS will manage jpeg. If you are shooting in RAW the program to read it in is very important, and not all programmes come for all OSs. There is something about Macs and Canons - the best RAW software seems to come only for windows.
2) More megapixels are not better, in fact they are probably worse in compact cameras, because noise rises as the pixel size gets smaller. You will see this particularly in low light conditions.
3) There are two reasons why SLRs are better, and a lot better, than anything else, if quality is what matters. One is size of sensor. They have big sensors, less noise. The other is that they do not use power to deliver the view through the viewfinder. This means more rapid operation, less power consumption. The difference between the alleged semi-pro small sensor cameras and true SLRs is huge, and way out of proportion to the difference either in cost or size. I think his view is, though he doesn't say this explicitly, that the 'semi pro' ones which look like SLRs but are not, is like lipstick on a pig. The only thing to use, if you are serious about picture quality, is an SLR. The difference, he says, is huge.
4) When considering SLRs, worry about dust. Every time you do a lens change, dust will be attracted to the sensor. Cleaning up the dust in photoshop or equivalent will be inredibly time consuming. He uses these special brushes,very expensive, to get the dust off the sensor. There are different ways of getting the dust off automatically now with different makes. Worry about dust. Worry about how you are going to manage, how much its going to cost, whether you have to return for maintenance to get it done. Think hard about whether you actually need to change lenses, and if not, get the kind of lens that is flexible enough to leave in place forever.
If you're a pro you can't avoid changing lenses. As an amateur, plan so you don't have to.
5) More optical zoom is not better. It is impossible to make 12 x optical zoom lenses that are as good at any given zoom as a non-zooming lens. So anything more than 4 or 5 x zoom means compromising quality at all zoom levels.
5) Image stabilisation is fine, but beware of processing that increases sharpness. What you want is a stabilised image, with as little processing as possible. If any sharpening is going to be done, do it yourself on the PC. Otherwise you risk ending up with a pre-processed image where further processing will just degrade quality.
6) He has cameras that cost 5k plus. Nevertheless, the ones that he is surprised to find give him excellent shots and surprisingly good quality are the 200 pound or so compacts with moderate MP and Zoom count. He particularly mentions a Fuji whose model number I have forgotten. It had quite a high MP count. I can find out if you want.
7) Some of these, particularly the more expensive ones, have all kinds of manual controls. He has been surprised to find that they don't appeal to him at all, though in SLRs he is very manual control oriented. He actually prefers simple point and click in compacts, and doesn't quite know why the ones which allow him the freedom he has with his expensive SLRs always seem to get left at home.
I found these conversations interesting because they are so different from anything you read in reviews. As a consequence I bought for my nearest and dearest a then high end 3 x zoom 4MP camera, a couple of large compact flash cards, and a reader. It has worked brilliantly. Its easy to use, the quality is excellent, the zoom is about right, its purse or jacket pocket size, and it goes everywhere. Its certainly not what I had in mind before I asked him.
Peter