Can I delete my local photo library now that all my photos are on iCloud?

Yes - if you trust Apple to never lose the photos, you can delete all your local copies.

Pay attention to which devices have full resolution images and which have "optimized" settings.

Also, I've been doing this for iTunes Match for some time. I have only lost a few dozen songs in two years of trusting the cloud. For that reason, I now set alarms every 6 months to download a local copy of all the songs from iCloud and back that up to an external HD. I'll be doing the same with my iCloud photo library to be sure I never lose all the photos.

I would add, that if the thought of losing a dozen photos or having to wait several weeks if your account should get messed up for engineering to work out the problem isn't your cup of tea, you might keep your originals on hard drives and in filesystems you know you have the tools to work with directly.

I thought that Apple had solved the local storage problem and made it simpler, but they seem to have repeated some prior problems of iPhoto Libraries. First, it appears you can indeed delete all your new local Photo Library items. However, when you want to look at them in Photos on Mac again it will re-download them all again. Apple says (https://support.apple.com/kb/HT204264) that using Optimize Storage setting makes this all fine but if you have 10 years of photos, even in some optimized format I think they will take up tons of space. I just started loading a few months and my local "Optimized Storage" is 2.5 GB already.

Taking a look on the Finder, I see Apple is again creating some type of Packaged folder with all the library items and tons of metadata. This is the same thing they did that was problematic for iPhotos and caused lots of people to create multiple libraries through (hidden features to switch libraries, initially) and have to move libraries around between local and network storage to deal with space limitations.

Now, Apple seems to imply that the Optimized Storage feature is smart enough to 'manage' my local storage for me. However, I have no controls to tell it to limit its use to, say, 10 GB. We will all find out soon just how good it is at managing this.