TPM has an AP story that the National Archives is moving ahead with attempts to use state-of-the-art technology to recover missing notes from Bob Haldeman that correspond to the famous 18 1/2 minute gap in a Watergate tape. This appears to be a follow-up to the initiative described in this MJ article from David Corn. Corn's article is good for the technical stuff.
As for what's at stake, I'm just going to recycle what I said about it back in July. The question is: what was in the tape that caused Nixon to destroy it? (It's overwhelmingly likely that it was deliberately destroyed, probably by Nixon, although Haldeman did have custody of some of the tapes at one point, and it now appears that Haldeman destroyed his own notes -- the notes that the Archives are trying to recover). There are three broad categories of things that Nixon would have wanted to destroy:
1. Evidence of the cover-up. Most of the last year of Watergate was about the cover-up of the cover-up, and Nixon may have wanted to destroy evidence of the cover-up. We know pretty much the whole story on this, although extra details are always welcome.
2. Prior knowledge of the break-in. There is no direct evidence that Nixon had prior knowledge, and for that matter Haldeman always denied prior knowledge (the evidence is that he certainly knew about the general program at the Committee to Re-Elect, but IIRC there's no specific finding that he clearly knew about the break-in in advance). This would be a major bombshell.
3. Other stuff. There are all kinds of wacky conspiracy theories out there, none of which are of great interest to me, but what I do think is possible is that there may be additional operations of the Plumbers that have never come out -- after the break-in, a lot of people destroyed a lot of evidence (although they also failed to destroy some stuff), and it's by no means clear that the Plumbers ever decided to tell all. As with #1, new details won't change the story in any important ways, but might be way fun anyway.
The AP story says we should hear more in a few months. I'll just add that the contents of the 18 1/2 minute gap is probably the most important known unknown about Watergate, but there could be other things that were successfully covered up that we won't ever know that we don't know, unless Liddy, Colson, Krogh, or Young have remaining unspilled beans they decide to let us in on.