Consider at least three things: sensor, "capability enhancement", and lens
1. Sensor
Sensor Type. Most are CCD/CMOS-based sensor implementing the Bayer filter. Only alternative to Bayer is Sigma Foveon sensor. Do some reading to know what meets your needs for today and tomorrow, or maybe just for today.
Resolution/megapixels - again almost all are equivalent
Conclusion here is most DSLR gives equivalent performance here and thus will not be a major resolving/distinguishing criteria.
2. "Capability Enhancements": These are things put into the body to assist or to enable your photography. Some more important ones are:
Autofocus (not applicable if you rely more on your eye to manually focus):
- Minimum contrast: Most autofocus are passive autofocus where contrast is the key factor in the algo, with some needing more or less contrast (Note: the lens used affects autofocus capability.)
- Accuracy, ie it focuses where you intend to focus. Multipoint autofocus do better here, as in single point AF you need to offset and hope your DOF is sufficient.
- Speed, also continuous AF
Exposure Metering (again not applicable if you meter manually with your eye)
- Accuracy and availability of various metering modes, eg spot, but I think this will not be a distinguishing factor with most DSLRs.
IS/VR: useful for low shutter capture, expands your handheld capability envelope
Noise Level/Reduction: Some are better, some are less so. Can be a distinguishing feature.
Others:
- Live View. Makes it more convenient for some kinds of creative photography.
- Dust Reduction/Removal. An indirect capability, as it eases maintenance, and so more up time for the camera and less maintenance expenses.
Lens: I think this is the most important factor. Body come and go, but lens almost always stay, and they can be more expensive than the body by 2 times or more.
Quality. Most OEM lens are superb but you pay for them if you want the quality; and also at that level, prices are also about the same for any camera brand.
Third Party Lens. Canon and Nikon by being most popular have the largest third party support, which means cheaper lens. Quality-wise they may or may not approach that of the OEM's.
So I suggest you work backwards, ie dont think of body first, but of the lens and work back to some compatible body, available today. But if you are complete newbie to SLR photography then this backward route maybe hard.
But a start point is to know what you want to shoot, eg if you want to shoot insects, you need macro lens, birds, then super long telephoto, sceneries, then wide angle, etc. And then why are you shooting? To make nice pictures of your girlfriend or boyfriend, to sell the pictures, or just for web use? Or maybe just to acquire an SLR experience?
OK, to be realistic, most of us didnt start this way too. This is speaking from hindsight. My experience was getting something with most bang for the buck, and then realised I'm stuck cos I'm hooked on the lens.
And then again, what camera you used may not really matter anymore these days, for almost any camera can do any shot you want to do. The "critical path" or "weakest link" right now is more in the photographer than in the camera.
As a parable, a good carpenter can make any furniture with just a single chisel, albeit not just any carpenter can do that; but someone without any idea what a chair is can do nothing, or more accurately, nothing novel or creative, even if he has latest computerised controlled, knowledge-embedded, AI assisted, 24/7 helpline, all-in-one furniture making machine.