Under these circumstances, the talk of the "reintroduction" of Activities becomes understandable. For some users, I suspect that the beta might be their first introduction to Activities.
Besides the sliding panel, KDE 4.5 added other efforts to make Activities more noticeable, such as an Activities Bar widget for switching between them. However, these efforts are minor compared the overhaul of Activities in the 4.6 beta.
Unfortunately, the 4.6 beta still includes both Virtual Desktops and Activities. Otherwise, though, Activities are more versatile. Activities could already take different Folder Views, allowing each to maintain its own set of icons, but now each Activity can have its own set of widgets. The release announcement also mentions being able to start and stop applications as you switch to and from an Activity, although that feature is apparently unimplemented in the beta.
To further encourage the use of Activities, their panel now includes the option to clone the current Activity, or to either choose an existing template, or to download a new one. In addition, Activities on the panel now have a set of mini-icons for removing or configuring Activities, making them more closely resemblance icons elsewhere on the desktop. All these changes should go a long way to helping Activities become part of users' awareness at last.
KDE 4.6 falls short of the dramatic changes that characterized the early releases in the KDE 4 series. All the same, its additions show KDE developers laying the groundwork for future development, and making changes to enrich the user experience. The changes to Activities are especially interesting, since they seem to show the development team learning from its mistakes and trying to correct them.
The early buzz is that 4.6 will become the defining version in the KDE 4 series. Personally, I would not go so far -- not with the potential that the behind-the-scenes improvements hold for future features. Undeniably, though, the release shows KDE finding new directions, and for the first time in a year, I find myself looking forward to an official KDE release.