Yes, ECC memory. The CPU also needs to be able to support it. Then there’s number of cores, unless you specifically need high single-core performance. Efficient and reliable PSU, low power-consumption, lots of memory. Stuff like that.

None of which are essential, BTW. Any working PC can be made into a server regardless of its hardware.

All server really is is just another PC that’s been built with a specific purpose in mind. The rest is software configuration.

Even your router is the same. It’s all computers.