I just got my set of NSDI papers to review, and have been looking them over.
One thing that immediately strikes me as I give them a first quick pass is how nice it is that the submissions are 14 double-column pages. The authors have space to present a meaningful introduction and a reasonably full description of and comparison with related work. They can include (detailed) pseudocode as well as description of their algorithms. They have space for a full page (or more) of graphs for their experimental results, and even more space to actually explain them. The papers actually make sense, in that they're written in sequential order without having to flip over to "appendices" to find results. The phrase "this will appear in the full paper" appears rarely -- not at all in most papers. The papers are, as a consequence, a pleasure to read. (Well, I can't vouch for the actual content yet, but you get what I mean.)
As a reviewer, it's also nice that if I tell the authors they've left out something that I think is important, I'll generally have confidence they'll have space to put it in if the paper is accepted, and that it's a reasonable complaint to make, in that they ostensibly had space to cover my issue. (There are some papers which fill the 14 pages and perhaps won't have something obvious that could be removed, but experience suggests they'll be rare.)
So I wonder again why theory conferences have 10-page single-column submission formats ("appendices allowed"!), or, even worse, for conferences like ICALP and ESA, they have final page counts of 10-12 pages in the over-1/2-blank-page LNCS format. (Really, it's just about enough space for an abstract, introduction, and pointer to your arxiv version.) Interestingly, for my accepted SODA papers this year -- which went with 10 page submissions, "full versions" attached on the back, but had 20 pages for accepted papers -- both sets of co-authors didn't want to bother when the final submission deadline came around to filling the 20 pages, figuring people could just be pointed to the arxiv (or eventual final journal) version. Why create yet another version of the paper according to arbitrary page limitations? I certainly couldn't suggest a good reason.
On the theory side, as I've maintained for years, we're doing something wrong with our submissions, with artificial page limits creating more mindless work for authors and making decisions more arbitrary than they need to be.