How to start a slideshow on the Mac quickly and easily?
In Leopard and later versions (up to El Capitan), select the files you want in the Finder and press the spacebar to invoke QuickLook. You can then use the arrow keys to cycle through them. If you click the full-screen icon, Finder and QuickLook displays the slideshow. You get extra animation in some os versions if Finder is showing files in cover-flow layout. 

In OS X Yosemite 10.10 and later, the maximise (full-screen) button is in the upper left-hand corner. Then press "play" to start the slideshow.

For OS X 10.9 to 10.7 the full screen icon is top right. 

In Tiger, there is a "Slideshow" option in the context menu in the Finder.

In Panther and earlier, it is indeed several steps before you can get a slideshow.