The image quality is one of the most important aspects of every camera: even the most advanced camera of the world wouldn't be a good choice if it had poor image quality. The best way to know the image quality is to try the camera; if it is not possible, you can read reviews and download image samples from reliable websites, as the excellent DPreview.  The various aspects of IQ are described in-depth in other articles, but here I'll summarize the three things to look at.

The resolution determines the level of detail and the maximum print size of the image. The number of megapixels gives an approximate idea of resolution, even though you need to use more precise measurements to know the actual resolution. Tha said, nearly all cameras currently available have enough resolution, both for amateur and professional.

The noise is an artefact that becomes more visibile at high ISO. A good camera must be usable throughout the entire range from ISO 100 to 1600; obviously, the highest ISO sensitivities are more noisy, but the noise and detail must be acceptable even at ISO 1600.

The dynamic range is the range of brightness values that can be

The sensor size is a very important aspect of the camera. Currently, there are mainly two sizes: the "FullFrame" (24x36mm) and the APS-C (16x24mm). They have both advantages and disadvantages.

The APS-C sensor are cheaper, some SLRs with APS-C sensor cost less than $ 500, while FF is more expensive (currently the cheapest FF camera is the Canon 5D, that costs $ 2,200). 

The APS-C sensors have a "focal lenght multiplier" that ranges from 1.5x to 2.0x, that is great for wildlife and distant subjects: a 70-200mm f/2.8 on an APS-C camera gives the same angle of view of a 105-300mm f/2.8 (70-200 x 1.5). Moreover, the "crop" of the sensor uses only the center portion of the image circle, that is the best area in terms of sharpness and lack of artefacts.

The APS-C sensors allow to design smaller and brighter lenses than Fullframe : for

The autofocus varies widely depending by the camera. Other than "Single AF", it is essential to have a "continuous AF", i.e. an AF system that tracks the subject as long as you keep helf-pressed the shutter release. You should be able to select the type of AF in every exposure Mode (P,S,A,M); avoid cameras that limit the continuous AF to the "Sport mode."

The number of AF points goes from three to the 51 points of the Nikon D3 series cameras, but it is not an indicator of the efficiency of AF; an AF with 10-15 areas might be as good as an AF with 40 or 50 areas. (Personally, I really like the 9 areas AF module of the Canon 40D)

There are two types of AF sensors: line and cross sensor. The cross sensor are more precise and

Any serious camera must have the four essential exposure modes: P (Program), S ot Tv (Shutter priority), A or Av (Aperture priority) and M (Manual). I use Av for the 90% or my photos, and M for the remaining 10%: these modes are essential because they allows to control some of the most important aspects of the image, the exposure and the depth of field. The "scene modes" (landscapes, portrait, macro, ect) that are often found in consumer cameras, insted, are pretty useless.

Other than that, it is important to have an "Evaluative" or "Matrix" meter, that calculates the exposure analyzing the entire image with advanced algorithms. I don't consider essential the other types of meter, as center-weighted or spot, since Evaluative allows to get almost always good exposures once you learn to use it properly, in conjunction with the histogram.

In many situations, you have to compensate the exposure. The best cameras allows to compensate the exposure in steps of 1/3 stop, in a range of +/- 5 stops. In the majority of situations a range of +/- 2 stop is enough, but in some particular scenes you need to apply

t is essential to have a wide range of shutter speed, at least from 30" to 1/4000, plus "B" (bulb) exposure. 

The longest speeds are very useful for landscapes: in early morning or late afternoon, the best hours for landscapes, the light is very dim, and you need long exposures (of course, you should use the lowest ISO setting and a sturdy tripod). In some situation, you might even need the B pose, to take exposures of minutes: in this case, it is essential to have a remote release, to avoid the vibrations caused by the shutter release.

The fast shutter speeds are useful for action and wildlife: if you like the photograph the animals in action, you need very fast SS to "freeze" the movements. Using wide apertures (f/4 or f/5.6) and ISO 400 I often get speeds between 1/1600 and 1/4000, that allows to create amazing action-stopping images. 

You should pay attention also to the sincro flash, that is the fastest shutter speed that you can use with the flash (at full power). A fast sincro flash is better because it allows to use flash even in bright light, to lighten the shadows: the best cameras have a sincro of 1/250. 

The first digital SLRs had small, dim screens: now the technology has noticeably improved, and the best DSLR have 3" LCD with a resolution ot 230,000 or even 900,000 pixels. The viewing angle has improved a lot, too: the most recent LCDs have a viewing angle of 160 degrees; they are very bright and easy to read even in direct sunlight.

Othen than a good LCD, there are some reviewing tools that are essential: the most important is the Histogram, a graph of the exposure. Once you have learnt to read the histogram, you can judge the exposure at a glance and you will rarely miss a photo due to bad exposure. Some cameras offers even RGB histograms: while they are not essential as the luminance histogram, they might be useful in some situation.

The highlight screen shows the overexposed areas as flashing highlights, and it complements the histogram. Some cameras have

If you plan to use often the camera under the rain, in dusty or saindy environments, or in other "unfriendly" environments, I'd recommend to pay attention to the sturdiness of the camera body and to weather sealing. The majority of semi-pro and professional bodies are pretty sturdy and well built, but only the top professional cameras (as the Canon 1 series) are weather sealed, i.e. they have rubber rings around every button (inside the boby), around the lens mount and in other parts of the body.

Weather seaded camera are not waterproof, you can not use them underwater, but they can be used without worries under the rain and in other extreme situations. Remember that you need a weather sealed lens (as many Canon L lenses) to get a complete sealing.

The size and the weight of the body varies considerably: some consumer bodies, as the Canon 400D, are very small and light, while the professional 1 series are big and bulky. If a small camera body is important for you, I'd suggest to consider a consumer or mid-range body as the Canon 40D and Canon 5D. On the other hand, a large body often has a better handling, and it has a second shutter release for vertical format - very useful if you often shoot in portrait format (or course, if the vertical grip is important for you, you can always add the optional battery grip on the smaller bodies).

The menu system should be well organized and easy to navigate, and the most important functions should be readily available through the buttons on the camera body or through the main menu. The camera must have RAW file format (but nowadays all SLRs offers RAW, only the cheap digicams are limited to JPEG). The other essential functions include mirror lock up (thats locks up the mirror some seconds before the actual exposure; it allows to avoid the blur caused by the mirror slap at slow shutter speeds) and self timer or remote control, very important to avoid the vibrations of shutter release at slow shutter speeds. 

If you photograph action and wildlife, it is important to have a fast continuous shooting: the best cameras are able to take about 30 RAW photos at 10 FPS. Many animals moves so fast that it is almost impossible to press the shutter at the right istant, even if you have fast reflexes. Shooting at 10 FPS allows to capture the action, and with digital you don't have to worry about the cost of film: I use a lot the continuos shooting for wildlife photography.

Nearly all new cameras incorporated a "dust reduction" function, that shakes the sensor or a filter placed in front of the sensor to eliminate the dust. I don't consider it essential (it is quite easy to remove the dust even with an old camera without automatic dust reduction), but it is a nice addition that allows to save some time and to change lenses without worrying about dust on the sensor.