I'm a kind of IOS specialist but I don't know anything about Android. I will probably receive my Nexus 5 tomorrow and my question is: What should I know about android ?
1) You no longer have to use the vendor default. You can use any app for any default. Chrome is very good, but if you want to default to firefox instead, Android lets you do that.
2) The UI is what you want it to be. Explore other launchers and decide which UI you like best. There are even launchers that will replicate iOS. You stand to lose nothing...they all operate independently. If you decide you don't like it, you can just uninstall it, and your original launcher is restored exactly how you left it.
What are the "must have" apps ?
Dropbox/box.com
Google Drive
A file manager (Astro is a good free option...Solid Explorer is also good)
Everything else is either pre-installed or a matter of opinion. I think everyone would use the apps above though.
My iPhones and iPads are Jailbroken .....
Rooting is more involved than jailbreaking. But most of the things you had to jailbreak for on the iPhone, you can already do by default on Android. That is why most people do not root on Android. There is no reason to most of the time.
Do I have to "root" my nexus ?
You do not need to root, and most people don't. You should only root if you have a specific reason to root (like an app or function that you can't do without rooting). I am an advanced user and have been rooted before, but so far have not had a reason to root the Nexus 5.
Can I install "cracked apps" ?
You can install whatever you want. Installing stuff not on the official market is called "sideloading". You need to go into your security settings to uncheck a box. But once you do that, you can install any APK file you want, or even use alternative markets like Amazon. None of this will interfere with the official market...that will work the same as it always did.
Google will not peek over your shoulder like Apple did. You can generally install whatever you want, just like you can on a PC.