Here's my own limited perspective on how Google+ apps will offer new opportunities for budding entrepreneurs. The release of such a platform is just a matter of time, and will very likely be based on OpenSocial. When a new platform gets launched, the bar will be very low for the kinds of apps that will succeed. Of course Google will have learned from the experiences of MySpace and Facebook and will limit the viral channels more than they did even initially, but still success in the beginning should be a bit easier.
Why do I feel qualified to write about this? I am probably not the most qualified person on the planet, but I did have some successful apps on both MySpace and Facebook when their platforms launched. On Facebook I was a few months late from the initial launch, but still managed to whip together something that got nearly 10 million installs. On MySpace I had an app that still remains in the top 20 with over 5 million installs, although that does not mean much as the platform is all but dead now.
The kinds of things I expect to succeed on the inevitable Google+ platform include anything that has some sort of viral component that is effective enough to ensure that the app will spread widely, but not blatant enough to get banned or secretly de-emphasized by Google. At first the userbase will be very affluent and geeky, so things like friend graph visualizers might be popular. As more and more "normal" people join the network, things like receiving points for recruiting friends to the app will gain in popularity. Existing games will reuse their brands and assets, FarmVille will launch on Google+ and getting points for recruits will be less obvious and will be branded as "neighboring" your friends instead of just plainly inviting them.
There will be Q&A and quiz apps that will become immensely popular, unless these are features that will get integrated into Google+ before third parties have a chance to have their own takes on them. If there is an images API, then many apps that do some sort of transformations on user profile pictures will be popular. Causes will probably launch on Google+ and top the charts, and remain there as everyone loves to be charitable. They will remain uncontested, as making such an app will require connections with charities that most app developers will not have, unless there is a Google.org app in there.
Most of the money will be again made with social games. Some of these will not require a massive up-front investment. There will be some games that are more text-based and will profit from users paying with virtual currency. Google+ users will be more professional and less casual, so these will not be as popular as similar games on Facebook, but there will be some developers that will profit enough to make a living off of them.
There will be an app that allows you to anonymously answer questions about your friends. This app will be quite popular and make a lot of money on AdSense. How to profit from Google+? Read up on OpenSocial, refresh Hacker News until you see the platform release announcement and then code like mad.