I don't see why Coria won't be a thread indoors if he can be a threat on hardcourts. He has won an indoor title, afterall. And no, he didn't play the final because Nalbandian had to withdraw, but a title is a title.
> what is it that makes him so formidable apart from his speed.
As CC pointed out, he's consistent, he's not a complete weakling, his aniticipation is incredible... but also, as they discussed on the NBC coverage, he moves really well. Also, imo, he "sees" the court in a way that most guys just don't... The whole chessmaster analogy. He sees angles and sets up points that you don't see just anybody do.
That said, the current match should be a lot closer than it is right now.