Sealed whey protein container past its use by date - is it still usable? I've had an accident that made me stop training for quite some time and I ended up with a sealed - never used nor opened - whey protein pot that expired Oct/2015.  

Considering it's a powder and the package was closed and sealed, I opened and it still seems fine, with the same texture and smell as expected. I tried it and the flavour is very good as well.

What issues I could have using it? Would the proteins be "gone"? Could I have health problems?
You didn't say where your product was packaged, but according to the USDA, "Use by" dates refer to best quality, and are not safety dates.  This is especially true when foods are handled well...in your case, factory sealed.  Additionally, I can't see how the proteins would go anywhere.  Given the reportedly good flavor (not an indication of safety, but simply of good taste), I would use it.