Interesting topic! The talk was very technical and also mostly theoretical. I would've liked some more examples during the talk, and existing examples should be a lot bigger on the slides.
Some of the terms and concepts that were new to me were explained as "similar to Java/C/etc". They were shown in an example later, but as I haven't worked with Java/C, those initial explanations were not very useful.
Overall I learned a lot about hhvm and hack, so well done!
First off, I think it is always a good thing when people share there insights. So, keep that up!
I expected al little more work/practical/from experience examples. So maybe next-time a little more of that.
It was good to see the pros and cons of HHVM and Hack. I am even more eager now to create some test projects with Hack. As I am always full of idea's and can be somewhat inconsequent, I love the idea of the strictness of Hack.
Props for the clearness and simplicity in which you explained various examples and the github is really nifty.
Great info about basics and advantages of HHVM & Hack.
Learned some great stuff last night.
Maybe some more comparison to PHP, but one can find that in the Github examples too.