First, a word of reassurance: there are no sweeping backwards-incompatible changes in wxWidgets 3.0 compared to 2.8. We did have to break compatibility in a few places but fixing the existing code to compile with 3.0 will be trivial, if needed at all, unlike 1.0 -> 2.0 transition which required rewriting it.
Second, the main change in 3.0 is the rethought support of Unicode. While wxWidgets supports Unicode since quite a long time, so far it did according to Win32 model: there were two different builds of the library, one ANSI and the other Unicode and you could either use the former to use the familiar char * strings but stay limited to a single encoding or choose the latter one and be able to use Unicode in the GUI but at the price of working with wchar_t * string only which implied using ugly-looking wxT() macro in a lot of places for example. wxWidgets 3.0 will have only one build (which on its own is a great simplification for developers, packagers and even users who won't hesitate between which one to choose) combining the best features of the two: it will always use Unicode internally but support char * string and simply convert them using the current encoding transparently. In other words, you will be able to write code in exactly the same way as with the old ANSI build if this is all you need but profit from the full Unicode support at the same time. This is especially nice for the existing wxWidgets applications which weren't always written with Unicode in mind and so often didn't even compile with the Unicode build of wx -- now they will and it will be possible to extend them to deal with Unicode at leisure instead of spending a big initial effort on wxT()-izing them first.
wxWidgets 3.0 is still work in progress, in particular documentation hasn't been updated to reflect the changes yet. However the API and the code are believed to be stable enough to be used and we'd welcome any feedback on the new API. In particular, please try recompiling the existing code with the version currently in the svn trunk and let us know about any problems you may encounter (other than those which are mentioned in docs/changes.txt: please read it first!).
There are, of course, many other new features in wxWidgets 3.0 (without speaking about the bug fixes...):
- The new wxDataViewCtrl class provides all the features lacking in wxListCtrl and a much better and simpler API to use them.
- There is a new wxDFB (DirectFB-based) port especially suitable for the embedded devices.
- wxGTK (and wxDFB) are also more efficient internally because they use UTF-8 for the string representation, instead of wchar_t and so avoid the conversions between the underlying toolkit strings and wxString.
- New and improved wxFileCtrl (thanks to Google summer of code).
- Many enhancements to wxRichTextCtrl.
- Important improvements to several controls under wxGTK, including wxToolBar, wxStaticText, wxHyperlinkCtrl and others.
- Support for auto-completion in wxTextCtrl and wxComboBox.