Instant Upload, for those who may not know yet, is probably the best feature available for casual Google+ users, because you can get value from it even if you don't really use Google+. What it does is automatically upload every picture you take on your device to your Google Photos (aka Picasa Web Albums) account in a private album. From there, you can sort out those photos into proper albums, or easily share a few over Google+. It makes it extremely easy to add photos to your G+ posts, but it is also a great tool if you happen to use Google as your online photo storage service.
The Google+ iOS app update that hit the iTunes App Store today not only adds Instant Upload, but has a couple extra features. It also now shows you who has +1ed a post, and it adds the new "What's Hot" stream, that Google has been trying to push as a way to surface cool content and users on G+.
Not a lot of new features, but definitely very good ones. If you don't have Google+ yet, you can grab the app for free in the iTunes App Store.