A lot of the features and functionality of so-called Web 2.0 sites are now common elements in most current web apps and sites. It's really gone beyond what was labelled 'Web 2.0' last year, because so many mainstream websites are now using these elements. It's no longer a niche trend. For your reference here is a summary of some of the popular elements in use by modern web sites and services:
Tagging is the process of labeling a piece of content It's gone beyond what was labelled 'Web 2.0' with metadata. Flickr and 43Things.com (a social goal-setting service that lets people enter their goals and share them with a community of people) were two early examples, but recently we've started to see tagging adopted by more mainstream companies such as Amazon. Tags are really just keywords, so there's no reason most websites can't utilize them more to help their users navigate.
Because user-generated content makes up such a large proportion of the data in many web apps these days, it is essential that there is an effective way to aggregate that data and extract the value. Yahoo! is probably the bigco to watch in regards to user content this year, although expect to also see more aggregation products from Google (e.g. extending Google Base) and Microsoft (as part of Vista) this year.
I also think we'll see a lot of little services - such as Personal Bee and Newsvine - ramping up aggregation functionality this year, perhaps giving services like Bloglines and Rojo something to worry about.
Filters and ranking
Web companies need to have efficient ways to filter content and rank according to criteria that reflects its usage. Amazon.com is a great example of the value of good filters and ranking systems. One way to create filters is to enable your users to review the content and rate it - Netflix.com is an example of this. Again, look out for a lot of innovation from smaller companies and services in 2006, which will help to spread this type of functionality to more and more websites.
RSS feeds are a common feature in web sites and services, because they allow users to subscribe to and get instant updates of content that interests them. Expect 2006 to be the year RSS gets taken to the mainstream and baked into the products and services of big companies like Microsoft and Yahoo. It'll also be the year when non-blog content and data gets turned into RSS on a much wider scale than we've seen before.
Early mash-ups were created by scraping data, either from a website or an RSS feed. A cleaner and more efficient method of using data from another application is to use an API. Companies that provide APIs to their data usually do so with terms and conditions attached, and often there are restrictions on how much data can be taken or how it is used. However it is less clear when mash-ups take data from an RSS feed or screen-scraping.
Social software features like tags and RSS, along with associated web functionality such as aggregation and filtering, are expanding into mainstream use rapidly now. Here's hoping the above elements are implemented into many more websites this year!