So, I see some complaining about lack of "apps" - really the best thing about Windows 8.1, is that it has a full web browser IE11. If there is any app you really need (sim card phone apps excluded), all you need to do is go to the website, and pin it to your start screen, some websites even support live tiles (wpcentral.com included).
That being said, for the most part, games seem to be the most used apps on other platforms. The beauty of the SP3, is that your not limited to mobile games. You can install emulators for game systems, you can install steam, and play full PC games (on low settings), you can even connect an Xbox controller. The store has a ton of games as well (gameloft seems to be launching their games cross platform now). Some of the crap games people are addicted to (candy crush, etc.) are not available, but you could always play the game through Facebook website (again, full PC).
Modern UI apps I recommend:
Filebrick - Modern File explorer, can tap into every cloud storage site I know of.
IM+ - for Google Chat if you don't want to use desktop
HereMaps
Hyper (amazing, you can download audio/video for offline playback)
Zillow (real estate listings)
PRadio (Pandora 3rd party app)
Outcoldplayer (stream your Google music through an app)
VLC (works pretty good now)
XBOX Music (stream free music, pretty much any album/song you want, can't see how anyone could complain about it)
Flipboard
Weave (similar to Flipboard)
Drawboard (amazing PDF markup app, works flawlessly with stylus)
Photoroom (decent photo editing)
fotor (decent photo editing)
WatchESPN
NFL Rewind
NFL Sunday Ticket
Sports Illustrated
OneCalendar (Google Calendar)
Freshpaint
Facebook