- Quick settings - If you are tired of doing Menu -> System Settings and scrolling, then this should be make things like disabling mobile data or switching to 2G/3G faster. Though one could argue you could get this using Power Toggles.
- Gesture keyboard - If you don't have Swype, then this should be handy. Though someone backported this feature to 4.0 and 4.1, so you still can get this. Jellybean 4.2 Keyboard
- New clock - I used to install Ultrachron for the stopwatch and timer. Now, it is built-in. The design is not as intuitive but you'll get used to it. It also looks so much better than Ultrachron.
- Font enhancement - There are slight tweaks to how text looks.
- Lock screen widgets - Frankly, I don't use this on the Nexus S. However, if you want to try the excellent DashClock lockscreen widget, you'll need 4.2. It is quite useful on the Nexus 7, where you have a larger screen.
If you really want the new features of 4.2, then the easiest way to upgrade will be to install Cyanogen 10.1. The Cyanogen wiki page has the best instructions IMHO. Read and reread that completely. The major steps to installing a custom ROM are the following:
- Unlock your bootloader - Download the Android SDK to be able to use the fastboot command to unlock the bootloader. This step wipes all data from your device so make sure you keep a backup of your photos and SMS messages.
- Install ClockworkMod recovery - Since your bootloader has already been unlocked, you can install software that will enable you to install a custom ROM. This is a small piece of software that you run by pressing Power Button and Volume Down at the same time when turning on your device.
- Install CyanogenMod - You'll be using ClockworkMod to install the custom ROM. Follow the instructions on wiping before installing the files. You'll need to install 2 files in sequence. The first file is about 150MB and will install Android 4.2. Choose the M1 if you want something stable. The nightly builds may have bugs and can randomly reboot but will have more advanced features like HDR. The second file provides Google Play and the Google apps.
- cm-10.1-20130121-EXPERIMENTAL-crespo-M1.zip or one of the nightlies (Update 2013-03-04: cm-10.1-20130304-EXPERIMENTAL-crespo-M2.zip has been released). See the Get CM page for the latest list.
Once you've done this, you'll be able to try out other alternative ROMs. Steps 1 and 2 no longer need to be done. Good luck on your journey in the land of custom ROMs!