I'm running 8.1 on a Lumia 1520, and while some of the hub integrations have been removed (photos, me, etc already mentioned), I haven't missed any of them. I run the Facebook Beta, which was just updated to increase integration with WP8.1. There is no issue with any of the sensors or radios (gps, bt, nfc, etc). In fact, most work better than ever, and finally AT&T users get Data Sense app as well as the new Storage Check and WiFi Sense.
The new features far outweigh the loss of some small degree of social integration. The Action Center is great and Cortana works great. I like being able to tell her "Tell my wife I'm on my way home" as I'm walking out the door at work, and she'll text her for me. In Cortana's Notebook, you define your inner circle, which allows you to assign aliases to contacts, so I don't have a contact called wife, it's aliased for Cortana.
I also like the lock screen themeing options, as well as the start page background. More and more apps have a transparent tile option, and there are apps now that let you override some apps that don't. The new Podcast app is great, too.
There are many more new features - I whole heartedly recommend installing this update as soon as possible. I upgraded both my Daughter and Granddaughters' Lumia 920s. Granddaughters went fine, but daughters phone required a hard reset, and she lost some pictures since she didn't have syncing to OneDrive (skydrive) turned on. Make sure you have all of the backup options enabled before you apply the update.