Some of the examples:

* Itanium has register windows.

* Itanium has register rotations, so that you can modulo-schedule a loop.

* Itanium has so many registers that a context switch is going to involve spilling several KB of memory.

* The main registers have "Not-a-Thing" values to be able to handle things like speculative loads that would have trapped. Handling this for register spills (or context switches!) appears to be "fun."

* It's a bi-endian architecture.

* The way you pack instructions in the EPIC encoding is... fun.

* The rules of how you can execute instructions mean that you kind of have branch delay slots, but not really.

* There are four floating-point environments because why not.

* Also, Itanium is predicated.

* The hints, oh god the hints. It feels like every time someone came up with an idea for a hint that might be useful to the processor, it was thrown in there. How is a compiler supposed to be able to generate all of these hints?

* It's an architecture that's complicated enough that you need to handwrite assembly to get good performance, but the assembly has enough arcane rules that handwriting assembly is unnecessarily difficult.