The 'Quality' of a camera means different things to different people. It depends on what you, personally, want/need from your camera.
You can take amazing photos with the entry-level D-SLR cameras with kit lenses from any of the manufacturers.
However, the top of the range cameras and lenses will allow you to create those images across a wider range of situations.
All D-SLR cameras are capable of producing horrible images if used poorly. The quality of the photographer is more important than the quality of camera. (Of course the quality of the photographer can be improved by taking advice, reading, taking LOTS of photos and examining them with a critical eye and feeding that back to future photos).
Some differences between entry-level and top-of-the-range cameras include:
- ability to produce clean images in low light
- sensor size (entry level cameras have smaller sensors than top-of-the-range, meaning lenses see a narrower/longer view but have poorer definition and scope for shallow depth of field)
- usability - it's often quicker to adjust manual settings on a top-of-the-range
- fastest shutter speed (some entry level cameras will shoot as fast as 1/2000s but upper models as fast as 1/8000s, giving you more options in very bright conditions or for freezing very quick motion)
- weatherproofness - professional cameras and lenses are better able to cope with dust or showers
- autofocus speed - professional cameras often focus quicker and more accurately on fast-moving subjects
- some camera bodies now include built-in Wi-Fi, GPS and/or control of wireless flashes, for those that value such things.