LiveJournal has a pretty neat "almost Web 2.0" feature that it now saves what you've typed every couple of minutes so that if your PC crashes or your browser goes bananas (as they sometimes do), that the post you've spent hours typing doesn't disappear into the ether. It seems to be working a little too well for me, though, as it just offered to restore the post that I successfully posted last night. Still, better safe than sorry.
This is the sort of incremental development, adding useful features to an existing site that I approve of. Unlike certain other blogging sites
that make a big thing of Web 2.0, but turn out to be big on style but somewhat weak on substance.
The next Web 2.0 add-on I'd like to see is a helper for the Tags box, also on the "Update Journal" page. This could be modelled on the address drop-down in GMail, which shows you people's email addresses as you type. When I start to type, I'd like a drop-down list to show a mix of tags I've used that start with the letters I type, combined with the most popular tags on LJ that start with those letters. As I type, the list would further refine itself. There should be something to distinguish tags I've used before from tags I haven't (perhaps colour), and something to indicate how much tags have been used (perhaps font size or a percentage bar). If I select a tag, it would complete it and add a comma so I can start typing another. I think this would be a relatively easy feature to add, but it would make organising posts a lot easier, and help avoid problems like some of my posts being tagged "Doctor Who" while others have "Dr Who".
I am writing a Web 2.0 app of my own, so I can see the point of using the techniques where they are useful and avoiding creating a load of unnecessary hype around it. Web 2.0 is useful, but it is not magic and it won't make your life complete (at least not on it's own).
Perhaps we should call sensible sites like LiveJournal "Web 1.75".