something that really stops or brakes your business,
has no immediate obvious solution,
seems like unsolvable or too complicated by most of your team.
This is the type of tasks I like to do, for example:
reverse engineering from assembler to C++,
refactoring of legacy systems,
design of universal code that helps to save time on maintenance and development.
I work with wide range of technologies from assembler to ASP and
always ready to learn new one if it is necessary for your task.