hey have all been sideshows (like the social stuff, for example)
The web has been social media since the first modem connected to a BBS, then Compuserve, AOL, FidoNet, etc. and forums leading all the way to Facebook. These aren't the sideshows, these have always been the main event from the very beginning for many people. Search is just a way to locate the information posted on those sideshows.
rather than anything disruptive which has enhanced the web but has not changed the core
Apps are the current major disruption to the web. Whether to build a web site vs. an app is a major make or break decision. The disruptive part which most webmasters don't seem to get is that computers and the internet have transitioned from programmable office tools and nerd toys into little handheld internet appliances that literally everyone now owns. So the issue becomes which is more acceptable to the masses with their handheld devices, the app or the website?
The further disruption IMO is actually convergence as everyone wants everything to work everywhere so search, GPS, social and communications is all converging and it's doing it at a breakneck pace. Companies are being forced by customer demand to open it's APIs to other programs to make it all happen otherwise customers will turn to some other company that does.
Prime example is OAuth, using your Facebook or Twitter login to instantly gain access to any site which just a few years ago would've been considered crazy and now everyone does it. Websites using this technology are literally losing control of the customer, which is a major paradigm shift. This is OK if you're a twitter add-on tool or Facebook app, but not really OK for an ecommerce site.