I think many people are overthinking the purpose behind all this. IBM sought out to create a machine that can understand natural language and answer questions based on Jeopardy format. What is really amazing is that they built a machine that understands (most) questions posed to it in natural language and is able to discern the appropriate answer rather than providing you with links to where a possible answer might be.
Since the format of the show is what it is, there is no way to level the playing field by adding a time buffer to when Watson is able to buzz in. The real neat part is that the machine 'knows' the answer at the same percentage as the humans (i.e about 85% of the time as was written above).
From my understanding of Jeopardy rules, you can only buzz in when Alex finishes reading the question....so that's sets the baseline and everyone starts at the same starting point. The machine also uses a buzzer in the same way that the humans do. The humans have advantage because they can read faster than Alex reads the question so there is no significant (most of the time) lag.
There was a question today where all three contestants did not buzz in (the humans did not know or were unsure, and Watson's confidence was not a the appropriate level). That in itself was very interesting.
If you did not know that Watson is a machine and did not see the show (rather just listened to it), Watson would pass the Turing test: http://en.wikipedia.org/wiki/Turing_test
That's the accomplishment here.