Most compelling has been Tory leader David Cameron's inability to close the deal. Though the press are now pretty solidly behind him, he has struggled to get a decisive lead.
That is the context behind Clegg's surprise performance in (our first) televised debate. The Labour spin doctors were saying that it was good for them that Clegg did well because he would take votes from the Tories - but now I think they are a bit more worried. Clegg's candidacy has the attraction of the magic third party protest vote: like Ross Perot, or Silvio Berlusconi, he gets the votes of the people who are fed up with the old 'ding dong' of left v right.
Whether he burns out like Perot, or soars like Berlusconi did back in the nineties, we'll have to wait and see.