There are some updates to watch for. Even reaching the mainstream press, the Java bug can affect systems running Snow Leopard (10.6) or later, making the Mac vulnerable to malware that may be encountered on a dodgy web page. It does not affect systems running 10.5 or older. To protect yourself, open Software Update and run any Security Updates and any Java Updates it offers.
Then open Safari's Preferences, Security button, and uncheck the box for Enable Java. You probably won't even miss it as legitimate sites that use it are dwindling in number. Should you visit a trusted site and it wants to use Java, just go back to Preferences, enable Java, then reload the page. Disable it when you are done.
In Chrome, there is no option to enable or disable it. Java version 7 is not compatible with Chrome, so if you have not deliberately installed the older Version 6 there is no Java running in Chrome. To find out how to do this, and read why it isn't a good idea, visit Apple's KB article.
In Firefox, go to the Tools menu and choose Add-Ons. When the window opens click the tab on the left called Plug-Ins. Look for Java Applet Plug-In in the list and click the Disable button. Reverse the process to enable it.
Another known weakness is in Adobe Flash. Safari disables older versions of it as soon as newer ones are released, so if your web page shows Disabled Plug-in instead of the video page you requested, click on those words to take you to Adobe and download and install the newest version. Systems older than 10.6 can't use the newest version so you may not be able to view some video pages. This is especially true in Tiger 10.4.11 or older. Adobe updates several times a year, so be sure to accept and install those updates. YouTube no longer requires Flash, but an awful lot of sites (such as FailBlog on the Cheezeburger Network) do use it so you can't give it up entirely.
Mountain Lion Crash Bug
There is a strange "Assertion bug" that will crash any app running under OSX 10.8.2. Mail, Safari, Messages, TextEdit, all of them. All you have to do is type File (with a capital F) followed by colon and 3 slashes - :///.
Don't do that. Goes boom every time. If you enjoy the gory details, they are at this CNet page. Some people have suggested it as a Mac Prank by sending the string from the Messages app on any smart phone to a Mac. Not only will it crash the Messages program upon receipt, it's archived so it will re-crash every time it's launched. To clear it out, follow these instructions.
The newest iMacs
I have now had some experience setting up both the 21.5" and the 27" models. As I reported previously, the smaller of the two is seriously constrained by the slow hard drive. However, Apple now offers the option of getting the Fusion drive in that model as well as the 27". If you are interested in that iMac, absolutely spring for the Fusion drive. The technology combines a 1-Tb or larger drive with a 128 Gb Solid State drive. The system software and your most-used applications are located on the SSD part of the system, and relocated to the mechanical part of the drive if you start using a different app more often.
To the user, it all looks like just one drive. You do not have to do anything to manage it. So all that remains as a problem with the new iMacs is the lack of an internal CD/DVD SuperDrive and the lack of a FireWire port. Both Macs have two Thunderbolt ports, so for $29 you can buy a T-Bolt to FireWire adapter and keep using your old external drives. The other port can be used as a video port for a 2nd monitor with a T-Bolt to DVI adapter, but very few Mac users need a second display with such a big screen as their primary.
iTunes 11; Mac Power Users
The newest iTunes is just short of a (needed) complete rewrite of the program. It has taken on so many non-music-related duties over the years that it has become a bloated mess. Unfortunately a few features have not made it into the new version, but overall it is easier to work with once you learn the new ways.
My favorite podcast and web site for all things Mac is called Mac Power Users. It's designed to help people become more proficient, rather than just focusing on the highly experienced. I subscribe to the show through iTunes in the Podcast section, then I put it on the iPod that lives in my car. If you do a lot of driving it's a great way to listen and learn, but there is no reason you can't just play it through your Mac.
The web page that supports the show is here (this episode that focuses on iTunes) and you can get the back catalog on iTunes and the web page for articles that support the information you hear. I highly recommend this show.