Beside hiphop not supporting namespaces and some other minor official PHP features, what I think is holding it back from major adoption is support for different platforms and architectures. For instance, work was done to make it function on 32bit platforms, but since the focus of main developers is 64 bit, the 32 bits support regressed and as far as I know it isn't usable. I'm not even mentioning ARM support which is nonexistent.
Many people would say arm support is not even needed but look at how ARM is reaching also the server market with products like this one: http://www.penguincomputing.com/Prod...tedServers/ARM Also there is a rise of popularity for small devices like the sheevaplug, raspberry pi, odroid, etc... which run latest versions of original PHP just fine (wish I could see hhvm too) and can become the new desktop computers of the near future.
Another issue is that hiphop only works on linux, no support for windows or mac. When all those platform and compatibility issues are addressed (if they ever do) hhvm will take out a lot of share from PHP original implementation. Maybe people would start taking the language more seriously and use it on areas it hasn't been much used up until now.
I appreciate what facebook developers have done, I see it as one of the most important contributions to the community from fb, I just wish it becomes as portable as original PHP, sometimes it can be frustrating seen a project like this one and wanting it to grow more, but not having the necessary knowledge to contribute back.