Extensions now work in incognito mode via the extensions manager, the bookmark manager has been tweaked, and Chrome 5 incorporates HTML5 features, such as Geolocation APIs, App Cache, web sockets, as well as file drag-and-drop. If you want to experience some of what HTML5 brings to the table, Google says you can drag and drop attachments in Gmail or enable the geolocation functionality in Google Maps.
Although Flash Player integration didn't make it into this build, it should come along with the full release of Flash 10.1, which isn't far off. If Chrome hasn't automatically updated yet, you can manually install version 5 by clicking the tools menu (looks like a wrench), then About Google Chrome. Complete installers are here: Windows, Mac, Linux.