What are the characteristics of a good user-defined data type?

List,

This is a general question for my own education. Something I read earlier
today triggered some ideas in my head, and out of curiosity I'm researching
and learning.

What characteristics of data sets lend themselves to reasonable user-defined
data types? Are there characteristics of a data type that would limit its
usefulness?

I've reviewed the PostgreSQL documentation on user defined data types. It
seems that data types that can't be ordered or compared for equality would
be bad candidates. After all, if a data type can't be indexed or used in a
where clause, what value does a custom type bring over a binary or textual
representation?

Additionally, the careful tone in the documentation regarding the definition
of the comparison and equality operators suggests that these definitions may
be an exceptionally delicate matter. Any experience or suggestions on the
matter?

Tim Hart