There are some things a good developer SHOULD know
Adi has posted a post that I am not in agreement with: there are some things a good developer is NOT required to know.
[Quoting Jeff Atwood and Peter Norvig] Know how long it takes your computer to execute an instruction, fetch a word from memory (with and without a cache miss), read consecutive words from disk, and seek to a new location on disk
I did learn these things, but after reading Jeff's post I have been trying to remember a project I worked on during the past 10 years which required this type of knowledge - and I got nothing.
Well, I used knowledge about reading & seeking to / from disks the last two weeks. They are extremely important when you are thinking about high perf large I/O. Fetching words from memory, and the implications of ensuring good locality have huge affects on the type of performance that you are getting to get. As a simple example, a large part of the reason that thread switching is slow is that the CPU cache is mostly misses when you switch a thread.
You should know those things, and you should know how to apply them. No, they don't come often, and I can think of a lot more things that I would like a developer to know (OO, to start with) more than those low level details, but those are important to know.