Recursion. In my view of an interactive system, information retrieval should be thought of as a process, not as a probe (as is the case with batch systems). With the exception of simple, explicit searches, the searcher is neither completely aware of what is contained in the database, nor confident of just which words to use in the query to elicit a desired response. Because of this, there needs to be a high degree of interaction between the searcher and the database to gain the desired outcome.
In my view, today's search systems have ample room for improvement when it comes to information interaction. Our technology has come so far, and yet the basic challenges of language and representation ceaselessly recur.