metering capabilities of pro SLRs are a big deal for field photographers. Cameras like the Nikon F1 and Canon EOS3+ have incredibly good metering capabilites and autofocus adjusting (The latest EOS cameras being able to track your eye movements very well and focus on what you're looking at, for the most part, in the viewer). I couldn't possibly go into explaning the details of metering abilities, but check out Popular Photography for a decent resource.
Ability to connect to more extravagant flash setups is a must for studio photographers (of course most sutdio photographers would rather use medium or large format). As light tweaking can make or break a picture.
Lenses are also make an incredible difference. For example, the $24,000 1200mm pro lenses with f-stops as wide as 2.0 (that only companies like national geographic and time would ever buy (loan) their employess) are incredible, allowing enough light to take a pic fast enough to stop motion. (atleast 1/500 sec IMHO) Lenses also differ in resolution, and image quality. Moreover, the ability to add filters to those lenses can have dramatic effect on the pics. (polarizers, filters for halogen, flourescent, etc. lighting)
SLRs can take pics at very high rates as well (15fps i think), allowing you better promise of getting the best picture.
The benefits to having an SLR (if you're serious about photography) over a simple point-and-shoot are endless. Though, many if not most serious photographers also use simple point-and-shoot (even disposable) cameras for many cases.