Portal plague is a memetic disease that some new media companies seem to suffer very badly of.
Typically, normally a company has a perfectly normal website that specializes in just one thing. Then, in an early phase of the disease, an acute featuritis hits the site. More and more features appear. The site slowly starts looking like a typical portal. In pathological cases banner ads fill the page and sometimes pop-up windows appear in numbers.
This disease is often lethal only for the company; Users of the website will only find this an inconvenience. Usually, portal features themselves aren't bad, but user interface bloat may lead to inconvenient look. A good cure is to burn the features and use minimalistic UI design.
A typical case would be AltaVista. AltaVista started out as a simple-looking search engine. Then they added features, tons of links and banner ads to their front page, while the search engine remained as useless as it was. People complained, but while AltaVista was the best search engine, that whining lead nowhere. Then Google came, with a simple no-bullshit user interface, non-obstructing ads, and search that actually works, and people quickly left AltaVista to rot. AltaVista tried to minimize damage by starting a simple-interface search engine Raging Search, but that probably came too late.