Pros & Cons of still running Mac OS X Snow Leopard?
Pros:

1) Speed. 10.6.x runs faster on older hardware, particularly if you only have a regular hard drive. I see the beachball much more often in Mountain Lion than I did in Snow Leopard because my MBPro only has a 5400-RPM drive and ML seems to access the hard disk a lot more. SSD is preferred for Lion and Mountain Lion.

2) Colored icons in the Finder sidebar. Apple made them grayscale in 10.7 and 10.8, a huge step back for usability. There is a hack that will re-enable color, but it can cause Finder instability.

3) Rosetta. If you upgrade to 10.7/10.8, you'll lose the ability to run apps based on PPC code.

Cons:

1) No iCloud. There are workarounds but I've never been able to get them to work properly or reliably.

2) Security updates. However, you can "stay safe" just by keeping Flash up-to-date (or uninstall it and only use Chrome for Flash), disabling Java in your web browsers, avoiding pirated software, running Little Snitch, and periodically running AV software.

These are the ones that stand out to me. I'm sure others can add to this list.