Maturation of social media: it's time for businesses to actually start getting value from social media. Developing useful services, products or even approaches could be very profitable. at the very least, you should know if, when and how to use social media for your business.
html5/css3/js frameworks: right now there are lots of technologies competing to be the platform for next generation web apps/sites. html5/css3 are around the corner and look very promising, even for very complex apps. We are also seeing how javascript frameworks are becoming absolutely necessary tools for everyday web development. Solutions such as node.js take js to a whole another level. These web standards might soon become powerful enough to compete with Flash for audio/video centric sites.
"New" languages: ruby and python continue to gain market shares while there are new promising languages such as Scala and Clojure.
Cloud computing: hate the term all you want but companies such as Amazon are revolutionizing hosting and computing. Also check of what hosting companies such as Linode or Slicehost are up to.
NoSQL: if you are working with very large data sets the various solutions offered by the NoSQL camp might be interesting. Be, however, cautious because it's all very new and challenging to use in production.
Game mechanics aka gamification: more and more sites are using typical game mechanics to create richer user experiences. Facebook, Zynga and Foursquare are the most successful companies doing this.
SEO: the biggest news on the SEO front is ... that it isn't news anymore. Everyone is doing it and so should you - from the very first day. Facebook and Twitter has had a huge impact on SEO and web marketing but the basics are the same: create great content/services and build relationships.
Web design: graphical trends come and go but the things that are here to stay are: a/b testing; using web standards (again html5/css3/js) instead of flash, images and other, less flexible/semantic technologies; better layout support for mobile devices; css frameworks such as Sass; CMS frameworks such as Drupal or Wordpress are commonplace, even for very demanding sites.