One of the biggest changes is the way the search works. Previously it would give separate results for installed apps, then settings, then files on your computer, drawing a blank if none of those got a hit. Now it lists results from all three of those plus Microsoft's own Bing search engine, alongside other categories that it doesn't consider to be "files", which includes music, emails, games and all sorts of other items you might have on your machine.
The search works much better this way, although the new Bing app isn't everyone's cup of tea. It's more visually appealing than in 8.0 but feels dumbed down, often giving you just three search results per page, sometimes preceeded by ads for content that Bing wants to push at you. For example, a search for "The Italian Job" brings up a full-screen front-end designed to persuade you to stream the 1969 movie via Microsoft's Xbox Video service. To get to the proper search results you have to swipe it off, but which way? Ah... no clues.
The search can also be pretty stupid at times. If you want to investigate the new recipe app that comes with the 8.1 update, searching for "food and drink" yields no results, because the app uses an "&" sign, not the word "and". Also, when you do find that app, even though you're presented with an icon in the search sidebar, it's not possible to drag this straight onto the Start screen. Instead you have to use a context-sensitive menu and choose "Pin to start".