Re: Must have apps
Nova launcher - Best, most configurable launcher out there
Tasker - Automate your phone in infinite ways (e.g., I have mine automatically turn on Wi-Fi at work or home, turn on bluetooth when I exit work or home, automatically launch certain apps when I enter my car, lower my ringer volume when at work, etc.)
Lux Dash - Adusts your screen brightness, works much better than the built-in ambient display, and more configurable.
Screebl Pro - Keeps your screen on as long as you hold your phone at a certain angle (e.g., holding and using your phone), but then turns the screen off when you set your phone down. I've been using it for years, I consider it essential.
Swiftkey - Best keyboard
Feedly
CamScanner - Best app for taking photos of documents/whiteboards/etc. and converting to legible PDFs, etc.
Evernote - Cross platform note taker, I use it extensively to clip websites and articles for reading offline, as well as taking notes, etc.
Gesture Search - Search through all your contacts quickly by just drawing the first letter or two of someone's name
LastPass - My favorite password manager, multi-platform, also allows you to use the fingerprint reader of the Nexus
Timely - Great alarm clock