Budget
You can easily spend thousands on a digital SLR with lenses and other accessories. Be honest about your budget. And remember that you'll also need a lens, memory, a protective filter, a camera bag, and extra batteries. You can save a lot of money by buying an entry-level digital SLR kit. Even entry-level digital SLRs like the Nikon D50 and Canon Digital Rebel XT are more powerful and deliver better image quality than an expensive compact digital camera. You might also consider saving on the camera body in order to invest in a better lens. Although camera features are nice, don't discount the importance of lens quality. Most photographers will be better served by buying a less expensive camera so that they can afford a pro lens.

The System
A digital SLR is a big investment. Most photographers will probably keep a digital SLR longer than a compact digital camera. So it's important to make your decision carefully. Besides the previously mentioned elements - speed, image quality, and flexibility - you need to consider the camera system that you're buying into. When you buy a digital SLR you're buying more than just a camera. You're also buying that manufacturer's camera design philosophy and their system of lenses and accessories. You're investing in the future of their digital SLR design. It's not a decision to be made casually. Look at the lenses available for the camera you like, look at the other digital SLR models that manufacturer offers. Do you see a system you can grow with? If not, you should probably look at another manufacturer.