a) Depends on the 3rd party application. Major ones like Photoshop are designed to behave the same regardless of platform. Some applications have minor differences in how they behave - notably 3D software. On the PC side of things, 3d professional applications are only supported with professional level graphics cards where as on the Mac the consumer GPU's get support. This is an arcane case where Apple's total benefits developers are there is little variation in official hardware and drivers. The flip side is also true where PC applications will get acceleration for certain features as Apple has not exposed that functionality through drivers (AMD's Crossfire for example).
b) Again, depends on the application. Many are simple self contained bundles that don't scatter random files throughout your storage. If they do, what they leave behind is generally benign they'll generally be data files stored in $HOME/Library/Application Support. The main exceptions are those pieces of software that interact with core parts of the system.
If you're wondering if there is an equivalent to Winrot, there isn't.
Dual Opteron 6376, 128 GB DDR3, Asus KGPE-D16, Radeon 6970
Mac Pro Dual Xeon E5645, 48 GB DDR3, GTX 770
Core i7 3930K@4.2 Ghz, 32 GB DDR3, GA-X79-UP5-Wifi
Core i7 2600K@4.4 Ghz, 16 GB DDR3, Radeon 6870, GA-X68XP-UD4