Image Quality
There are lots variables involved in image quality. Pixel count, pixel size, noise, and lens quality are the most basic and obvious factors.

• Pixel count and pixel size: Digital image quality is determined by a lot more than just the number of pixels a camera captures. Digital SLRs, with their large sensors, are the best illustration of this fact. If you compare an image from a 6-megapixel compact digital to a photo of the same subject taken with a digital SLR, the SLR image will have better image quality. That's because digital SLRs have larger pixels that capture images with less noise (digital grain), richer color, and less digital artifacting.
• ISO / Noise: Most compact digital cameras have an approximate sensitivity range of ISO 50 to 400. Most digital SLRs have a sensitivity range of ISO 100 to 1600. Higher ISO makes it possible to get good photos in low light situations or use faster shutter speeds to freeze action. But that's not all. Besides offering higher sensitivity, digital SLRs also deliver cleaner photos at high ISO settings. Anyone who's tried to use the ISO 400 setting with a compact digital camera knows how poor the quality usually is. Most digital SLRs offer excellent, low noise images up to ISO 400, and useable quality right up to ISO 800. This means you can use your digital SLR in a lot more situations than a compact. And you don't have to use the flash as much.
• Lens quality: This might seem obvious, but better lenses make better photos. A digital SLR offers the option of using the best quality optics and that means better image quality.

Speed
Speed is one of the main reasons people invest in digital SLRs. Digital SLR electronics are a lot more robust than those in compact digital cameras. They have faster processors, more buffer space, and faster capture rates.

• Shutter-lag: This is the time it takes for the camera to take a picture after you press the shutter-release button. One of the most common complaints from compact digital camera owners is that they "miss" too many shots. They press the shutter release but the picture is captured after the moment has passed. Digital SLRs are much, much faster. There is some variation and there's always some lag. But most people won't be able to detect it at all. The picture is taken just about exactly when you press the button.
• Start-up: This is the time it takes a camera to be ready for taking pictures, after you turn it on. More processor power makes digital SLRs able to completely start-up far more quickly than most compacts. Like shutter-lag, there is some variation between camera models. The most expensive pro digital SLRs are ready almost immediately after you turn them on. Entry-level digital SLRs might not be quite as fast. But they'll be faster than most compact digital cameras.
• Buffer: Think of the buffer as temporary image storage. The camera uses the buffer to store images while it's shooting. This allows the photographer to continue shooting even while the camera is processing images. It can store unprocessed images in the buffer until the processor is free. And you can keep on shooting until the buffer is full. High-speed cameras like the Canon EOS 1D Mark II N and the Nikon D2Hs have very large buffers so that they can have capture rates of 8 frames per second. As long as your camera buffer has space, there's no such thing as shot-to-shot time.