Hydraulic Vs. Mechanical disc brakes What is the difference between hydraulic and mechanical disc brakes? Are hydraulics more reliable? Or more powerful? I'm curious since, hydraulics seem to get more attention and seem to be on professional bikes.
Hydraulics are used on higher end systems, cables are often a sign of a cheap brake set, so your observations are correct regarding "professional" bikes. However there are very good cable disc brakes (e.g. Avid BB7's) that are the exception that proves the rule. . 

Cables have the disadvantage of friction that hydraulics virtually eliminate. It is significantly easier to modulate hydraulic brakes, you get more force delivered to the pads, hence more stopping power for the same input, and faster/more reliable and predictable pad retraction when releasing the brakes. 
As the Hydraulic system is sealed, grit and grime cannot get in to jam up the moving bits, making them virtually maintenance free (replace worn pads is about all need doing)
Hydraulic systems are also easier to set up and adjust (with the right tools - next to impossible without them). Cables have to the exact length, and need "tweaking" as they stretch from new.  

That said - a good quality and well maintained cable system will out perform a poor quality, cheap hydraulic brake set, so you cannot state "Its hydraulic therefore it's better".  

Disadvantages of hydraulics occur when you get a leak such as hole in a hose or blown seal. They are less "field serviceable" than cables, essentially the brake is rendered useless and unrepairable (in the field) by a minor fault - which fortunately happens very rarely, and usually caused by poor transport rather than while out riding. 

Another reported problem with hydraulics is boiling of fluid. This is more of a problem on road bikes where mountain descents of 100's of vertical meters in a very short time are more common. With overheated fluid initially the pressure in the system stops the fluid boiling. When the brake is released (even for a moment), the pressure comes off and fluid boils, and the brakes no longer work - at all - the lever just goes all the way in. Another thing that can happen is the rider stops with no problems, but the heat in the calliper (no longer being cooled by airflow) migrates to the fluid over the next few minutes, so when the rider rides off there are no brakes.  (This is different to disc fade where the pads and disc overheat and brakes loss effectiveness slowly - both types suffer this equally.) 

Watch out in the future for Hydraulic shifters.... Available now if you have the big $$$$, lighter weight than XT and XX level components, almost certainly more reliable and easier to setup.... (Update 2015- Electric shifters mean these will not become main stream.)