What makes Groovy unsuitable as a replacement for Java, if the performance is now comparable?

For the masses, imho, all round Groovy just isn't polished enough and is too complex. We are bringing someone up to speed now and there is an enormous amount to teach. There are a range of pitfalls that you have to alert them to. Mind you, a lot of other Java replacements are like that (eg. Scala).

Particularly though, the static compilation is just not comprehensive enough. It tries to do type inference but it's limited and invariably you end up having to add type casts in ugly places to convince it that expressions are valid. Occasionally it actually flat out refuses to compile something valid and you have to find a different way to do it. Sometimes it compiles with one version of groovy and then is broken in the next. It works great though to selectively add to hotspots of code and in other places to speed them up and define them better.

Having said that, we are essentially using Groovy as a java replacement and for our team I think the above costs are well worth it. This is partly because we have a heterogenous application that is partly dynamic in its nature and Groovy's dynamic scripting ability is amazing for this. The fact we can code our whole back end in the same language makes it super easy and well integrated.