The UI was completely rewritten, from an information dense, fast, easy to use interface that had grown organically as the NWS put more and more data online, to the "web 2.0" interface with gobs of white space, information removed or placed in hard-to-find areas, and the UI performance is less than one-fifth of what it was previously, even on modern computers with fast connections. It is a pale imitation of what it once was.
To their credit, you could still access weather underground "classic" by browsing to classic.wunderground.com, but in 2015 they finally shut that site down, shortly before selling it to IBM. Wunderground has only gone downhill since being bought out by IBM
Wunderground was an amazing website, the classic version is still the best weather website that the internet has ever had, but it's gone now, and the shambling corpse that still remains does not deserve any praise it gets at this point.
It's long been about halfway down my list to write a fully open source clone of classic wunderground, but I probably won't get to it this year. I have a bunch of screenshots/rendered CSS etc as source material if anyone is interested in taking this on.