When printing in Leopard, the print queue application remains open after the print job is done. In Tiger and all previous versions of OS X, the application would quit automatically as soon as the print job was finished. I find it very irritating to have to go and quit the application after every print job.
To stop this, after a print job, or if/when you have a printer open in the dock, right click it in the dock and choose auto quit. I did this with one printer and it changed the settings on all of mine.