Do you have pages in Google's supplemental index? Get 'em out of there!
As a reminder, supplemental results aren’t something to be afraid of; I’ve got pages from my site in the supplemental results, for example. A complete software rewrite of the infrastructure for supplemental results launched in Summer o’ 2005, and the supplemental results continue to get fresher. Having urls in the supplemental results doesn’t mean that you have some sort of penalty at all; the main determinant of whether a url is in our main web index or in the supplemental index is PageRank. If you used to have pages in our main web index and now they’re in the supplemental results, a good hypothesis is that we might not be counting links to your pages with the same weight as we have in the past. The approach I’d recommend in that case is to use solid white-hat SEO to get high-quality links (e.g. editorially given by other sites on the basis of merit).
Google is even considering removing the supplemental result tag. There won't be any way for us to tell if any page is supplemental.
Over time, the supplemental results are less and less supplemental and more and more likely to show up for any given query. As I mentioned at SMX Seattle, my personal preference would be to drop the "Supplemental Result" tag altogether because those results are 1) getting fresher and fresher, and 2) starting to show up more and more often for regular web searches. Especially as the supplemental results get more fresh, I'd like to leave that tag behind because it still has some negative connotations for people who remember the previous implementation of supplemental results (which has now mostly been replaced with a newer/better implementation).
If they ever remove the supplemental tag, I agree with Rand in that that information should be available via WebmasterCentral. Why? Supplemental pages are pages that do not have enough quality back links, and as such are not deserving enough to be listed in the main index.
As many webmasters have noted, web pages in the supplemental index do not send as much traffic as the pages in the main index. Let me give you the technical explanation for this. I found this out via a friendly exchange of comments with halfdek in searchengineland.com. He provided a direct quote from the Matt Cutts's Q&A video at SMX Seattle Advanced.
We parse pages and we index pages differently when they're in the supplemental index. Think of it almost as if its sort of a compressed summary. So we'll index some of the words in different ways on pages from the supplemental index, but not necessarily every single word in every single phrase relationship…
The reason why your pages in the supplemental index do not send as much traffic as the ones in the regular index is because Google only indexes a summary of those pages, instead of indexing the full content as it does for the regular index. In situations like these my series on Google's inner workings comes in handy. Take this example:
You've just finished up reading Seth Godin's popular book Unleashing the Ideavirus — a fascinating read. The book has a copious index at the end which is very useful for finding pages if you remember any of the funny words he came up with (hive, sneezer, etc.). Now, imagine your friend reads the book and has to write up a summary for a college report. Later he is asked to create an index of the book, but he only uses the words that are in his summary. Now he can find pages in the book by any of the words in his summary. Unfortunately, because there are fewer words in the summary, his index is paltry compared to the book's true index. You won't find "sneezer" in it for sure! Suddenly his ability to find pages is going to be more limited. The book's full index is simply more comprehensive. There are more words to use to find more pages.
In a similar fashion, having your pages in the supplemental index means that only a portion of the words are being indexed. There are many words and combinations that will not trigger your result in the search. I don't know about you, but if I write a 2000-word post, full of longtail keywords, I want the full page to be indexed. No summaries for me if you please.
Let's hope Google keeps the supplemental result tag. Knowing which of our pages are supplemental helps us promote them better and get more links to them.