Pros of rooting: Apps that require root. My biggest reason (initially before the days of my crack flash addiction
) was Titanium Backup. All my apps and data are always backed up and can be transferred anywhere. There are also some other neat things in TiBu. But that's only one example of a root app, the list is endless.
Cons of rooting: The only con I can think of is sometimes apps and SuperUser don't play nice. But it can easily not he a con if you go in to SU and deny the app SuperUser access. So no biggie
Pros of Unlocking: Personally this is the main reason I love Android. It's customisable thanks to unlocking. Through recovery, you can flash all sorts of stuff including custom ROMs, themes, kernels, and other mods. For example you can make your phone completely full screen (no softkeys) with a flashable mod. Also, NANDROIDS. That will make an entire image of how the phone is at the time of the backup. It is THE ultimate backup. Consider it a step up from TiBu
Cons of Unlocking: Screwing it up I guess is a downfall. Though you can always restore this phone back, so I wouldn't consider it an issue.
Pros of Custom ROMs: My FAVORITE part about Android. Custom ROMs allow you to make your phone function how YOU like, and look how YOU want it to look. No two Android phones must look the same. I guarantee my phone looks 100% different than yours and does many different things than yours though we're on the same phone. If we both had iPhones, our phones would be the same, just with different apps and wallpapers. Custom ROMs can add features to your phone that weren't there in the stock ROM. At the same time, you may only want one tweak, and keep the phone mostly stock looking. Specifically on GNex ROMs, many of them give you the OPTION to make it very close to stock or very different from stock. And that also depends on the ROM. I'm running AOKP which can allow me to do SO much more, perhaps more so than other ROMs, but some people may not want that, and they can install a different one with less features. Basically, it let's you make your phone how you want it to be.
Cons of Custom ROMs: Bugs, glitches, and instability. In all honesty though, I've tried almost every ROM out there, and some are slightly buggy and some have no bugs at all. It all depends on the developer and the community and the GNex has the best community and the best and most responsive developers ever. If there's a bug, they'll squash it quickly.
Pros of Custom Kernels: They can speed up your phone, improve battery life, and make your phone work either harder or less hard than it was designed to.
Cons of Custom Kernels: Kernels are probably the most dangerous thing to mess with. You can seriously mess up your phone by, for example, not giving it enough power to boot or overclocking it to a point where it's bad for the phone. However, have a nandroid backup and you should be fine. As long as you don't screw with it too much, it will produce satisfying and improved results with little issue.
I hope all of this helped
any questions about anything I've said or any specific questions about certain things or specific ROMs or kernels just let me know
Oh, and welcome to Android and Android Forums! And to the Galaxy Nexus community!