First of all, any of the major brands will perform well. So while it may not exactly aid the decision process, rest assured that there is no "wrong" choice. You will be able to take great photos no matter what you buy. With the basics taken care of, the real competition in your brain will be about details.
This will be the longest entry by far, but you need not read all of it. I have ordered the sections roughly by how important I think that quality is, the most important being first. If one of the factors is enough to drive a decision, stop reading--that's your brand!
(Technically, I'm covering interchangeable lens cameras in general, not strictly SLRs. This means I will mention the new mirrorless models, which I think are similar enough to real SLRs as far as my intended audience is concerned.)
If the camera is uncomfortable to hold, you will avoid using it. If it is too big and heavy, you will avoid carrying it. If it is too hard to control, you will simply give up on it. There should be as few obstacles as possible to using the camera.
There is no substitute for physically holding an SLR in your hands. Go to a retail store and try several models out. Manufacturers generally design their cameras similarly, so if the latest model isn't on demo, try last year's model. The layout will be more similar than not.
Specific things to look for:
- Too small. Cheaper SLR bodies and mirrorless bodies usually have less switches, are made of plastic, and use a dimmer pentamirror. If you have large hands or weak eyes, you may find these bodies uncomfortable to handle.
- Too big. Upscale SLR bodies tend to be made of metal rather than plastic, have a top LCD, and a glass pentaprism. This makes them bigger and heavier all around. If you have small hands, are physically weak, or plan to travel long distances with the camera, you may find these tiring to hold.
- Switch placement. Can you reach all the buttons without having to shift your grip or removing your eye from the viewfinder? Is the wheel(s) comfortable to use?
- Viewfinder: Is it comfortable to see through? Is the shot information laid out to your liking? If you wear glasses, is the eyepoint far enough that you don't have to mash your face and glasses right up against the camera (too badly)?
Do you carry your equipment on a cushion of crushed velvet? Or do you drop them on the ground and kick vigorously just in case?
Impact and weather are the enemy. Bodies range from unsealed plastic to sealed magnesium alloy. If you are a typical tourist more interested in Disneyland than the Middle East, plastic will work just fine. When going on a desert adventure or to the tundra, you will probably benefit from dust and water resistance.
Every brand offers weather sealed bodies, but generally they are high end models. Pentax is an exception, as some of their cheaper bodies are sealed as well. But sealed bodies must be paired with sealed lenses, or the lens mount just becomes a giant leak.
Weather-sealed does not mean waterproof. Buy a dedicated waterproof camera or waterproof housing if that is what you are really after.
Did you know your friends and family may have already decided for you?
SLRs are most useful with a library of lenses to fit a shooting situation. Building from scratch even a modest collection of (hundred- to thousand-dollar) lenses can be quite expensive. So inheriting a system or knowing a photographer who would be willing to loan equipment is a very strong incentive to use the same brand. As long as you can stand that brand's ergonomics of course.
Sony E and micro 4/3 cameras do not have a mirror. This means they can omit the mirror assembly and optical viewfinder, which makes them slimmer and lighter. You will only have the LCD and/or electronic viewfinder for framing. The framing lag will make following action much more difficult. The backlit screen may not be bright enough to use in broad daylight. Conversely, its brightness may impair your night vision when operating in low or no light. (Also, try spotting a constellation on an LCD...) Whether the trade off is worth it is highly personal. I will simply say that SLRs are more all-purpose.
For speculation, Nikon will likely be releasing their own mirrorless line, which will likely need a new mount to deliver on smaller total size. Canon announced their own intention to release a small body of comparable size to mirrorless cameras, but avoided saying that their product would be mirrorless also. (Historical note: Canon once made APS film SLRs of similar size as modern mirrorless bodies.)
Anti-shake systems are known by many names: Canon's IS, Nikon's VR, Sony's SteadyShot INSIDE, Panasonic's Mega OIS, Pentax's SR, etc. Such systems today are of two types: they either move part of the lens or move the camera's sensor.
- Canon, Nikon and Panasonic use lens systems. You will benefit from it only when using a specific lens designed with this feature, usually at extra cost. Kit lenses generally have this as standard. The optical viewfinder and autofocus system will both benefit from it, as the lens corrects the image before passing it to the rest of the camera.
- Sony, Olympus and Pentax use sensor shift systems. This is usually standard in their bodies. You will benefit from this when using any lens. However, because it is installed on the sensor, it only works when the sensor is in use. It will not stabilize the image for the viewfinder or autofocus system. Mirrorless cameras are an exception because they use the sensor full-time.
Companies are not created equal. The systems they create are not equal either. Each manufacturer offers a slightly different selection of lenses to set their brand apart.
While I emphasize unique and distinctive lenses here, I will point out that these are niche lenses targeting enthusiasts and professionals. The far more popular and useful "general purpose" lenses are made by every manufacturer. This comparison is largely academic.
- Canon is the market leader. They offer one of the largest lens selections around, including specialty lenses such as the tilt-shift lenses, an extreme macro lens (1-5x life size!), a "budget professional" f/4 lens series, and the largest selection of super telephoto lenses (over 300mm focal length).
- Nikon is the other market leader. They offer the other one of the largest lens selections around, including their own tilt-shift lenses, an unequaled wide angle zoom lens (14-24mm), and a pair of unique portrait lenses ("DC" for distortion control). Among their discontinued fare is a dedicated astrophotgraphy lens and a macro zoom lens capable of 0.75x magnification.
- Sony is the new Minolta. Their selection is smaller than Canon & Nikon but is still large, notably including a compact autofocusing 500mm mirror lens and an even more unique portrait lens ("STF" for smooth transition focus). Sony recently released a mirrorless camera line to compete with micro 4/3. There are only 2 lenses so far fitting this new E-mount.
- Olympus and Panasonic share the 4/3 ("Four-Thirds") mount and thus can share lenses. They are focusing on their new mirrorless cameras, which use the micro 4/3 mount. Micro 4/3 lenses are more compact than SLR lenses. Both standard and micro 4/3 have a more limited lens selection than Sony A and Pentax.
- Pentax is owned by Hoya, a major optical glass manufacturer. Their lens selection is comparable to Sony's. They are known for weather sealing even low grade lenses (other manufacturers force you to buy the professional lenses) and their compact and well-built "limited" prime lenses, among them a few exceptionally thin "pancake" lenses.
Because of their physical design, mirrorless cameras can accept adapters for many lenses from other manufacturers, including those designed for long dead/outdated cameras. You will lose automated functionality, but for some applications (macro and video come to mind) this would still be acceptable. Old manual focus lenses might actually be preferable, as their focus mechanism is designed for hand focusing rather than compared to autofocus lenses.
Similarly, Canon cameras can be fitted with adapters for other manufacturers' lenses as well, though the selection is far less. But notably, Nikon lenses will fit. As with mirrorless cameras, you will lose automated functionality.
Except for their high end models, Pentax cameras accept AA batteries natively. Other brands use proprietary lithium ion batteries and need external camera grips to accept AA.
If you care about what the pros use, Canon and Nikon currently split the professional market and will likely continue to do so for the foreseeable future. This trickles down to the SLR market as a whole. Consequently, these brands' secondhand, rental and dealer markets are much larger and more accessible than the others.
So... got a brand or at least some candidates yet? How to choose specific models of equipment is next.