A number of things prompted this post, the main catalyst was a question that came up at work and which I hope to provide and answer:
Partly it was also something I read on ReadWriteWeb, 5 Ways You Can Fall in Love With Tagging Again. Much of this wasn't new to me but one statement in particular One of the best things about tagging URLs is that all kinds of RSSfeeds become available. Made me think a little. The RSS feeds of tagged urls I have employed before and we have been using tags to identify interesting urls to work colleages for sometime. Why just restrict yourself to tagged urls though, anything that you can tag online could result in useful RSS feeds (although it is usually going to resolve to a useful url). In fact it is sometimes easy to forget that feeds from de.licio.us tags represent tagged urls (the comments are useful here to add further information), a feed directly from a set of tagged resources could in theory provide richer information.
Another element was the recent and very fruitful experiments I have been having with tagging and RSS feeds to help me with the process of learning Chinese vocabulary. I tag online dictionary entries on Chinese words as urls in del.icio.us, keeping the dictionary definition in the title and comment (along with other notes in the comment) and consuming the feeds in various places allows me to sort, search through, view and categorise the words I am learning in many different ways. Becasue these are not new items some ways of consuming RSS don't allow you easily remove items that have moved from one category to another, but using a combination of GoogleReader, RSSOwl, live bookmarks, Netvibes etc. I have all the ways to view, sort, search, archive my word lists that I could ever wish for.
Back to the orignal question, having access to RSS feeds of your tags in an online application allows you sort search and view your data in ways that suit you but that may not be supported in the original application. If you use two or more applications for similar purposes then RSS feeds can allow you to "silo bust" and combine similar information that you are interested in from many different sources.
Ouch unfortunately thinking about this has led to an uncomfortable conclusion, although del.icio.us is tagging links, these links represent resources.
If you are writing an application and providing tagging for your users then perhaps you should ask "how do these tags add value over them just tagging the resources in a service like del.icio.us?"
Perhaps one option is to co-opt your tagging systems to add other functionality to your site. By using a series of reserved "system tags" you can added a fileing cabinet of items for later reading or items for sharing etc. etc. In the same way that much new functionality is built on top of del.icio.us, (the readeroo plugin for example). If you already have data that maps tags to users why not use this to define other behaviours?