In particular: I have a forum app running on AWS, costs me about 150$/month, getting about 500,000 pageviews. Could I run this for free on Google Appengine?
1) Last I checked there was still no naked domain support. So if you plan on defaulting to http://mystartup.com/ instead of http://www.mystartup.com/, you might want to check if this is possible yet.
2) HTTPS support is limited to https://mystartup.appspot.com/. You CANNOT use https://www.mystartup.com/ last time I check. This is due to a technical limitation in SSL certificates that I'm not sure was considered at the time that Google designed the AppEngine architecture. There are ways around this, but it's messy and inelegant.
One of the great benefits of AppEngine is using it as a cache server, because if I remember correctly this is either free or really cheap. I forget the exact details of how we worked this out at my last startup, but it's worth looking into because using Google's global datacenters for caching is much much cheaper than paying for AWS bandwidth and you also get much lower latency times.
Do you have data/etc on the performance?
I will contact the devs I used to work with and ask them about that and let you know.
Assuming that is your daily pageviews, you could run that app for free on Google Appengine.
The only down side to Appengine in my experience is the cost of rewriting the app and migration, and performance issues (and cold start up times) related to Appengine's architecture.
AWS's main selling point is that you can programmaticly add and remove instances. If you are not using that, I'd consider migrating to a VPS such as slicehost or linode, where you should be able to run your forum as is.