I agree with Dr. Walkden here. While it was used as an interjection at times (just as it is today when someone exclaims, "What?!") in the context of the opening line of Beowulf "hwæt" is more likely being used to reformulate a statement in order to convey a sense of emphasis. An example in modern English would be something like, rather than saying "That was a gorgeous sunset!", one says "What a gorgeous sunset that was!". (Notice that the verb has now moved to the end of the sentence. In fact if you look at the last word of the line in question, we have the verb "fremedon" which means "performed", so indeed the placement of "what" at the beginning of the line facilitates the restructuring of the sentence in such a way that makes it "sounds right".)