Don't just press the red close button on the window and think the application has stopped. Learn to use 'Apple Cmd' + q to actually quit the application. When my wife first started running under OS X, she'd have dozens of applications up and running. That is the quickest way to think the system is bogging down.
The system generally sleeps well, but it is good to restart the computer about once a month. I've found that the more you use different wireless networks, the more important this becomes. We've had a number of systems not be restarted in months that were fine because they were always on the same network.
In general, the UNIX/Linux/BSD based systems are better at disk management and defragging than Windows. OS X underlying system is BSD based, not Linux.