Not to flog a dead equine, but the lack of a low-level Drawing API (like WPF, Flash, and other RIA frameworks offer) really limits the applicability of Silverlight to line of business applications (and games) that need large #'s of graphical elements/shapes.
A classic example would be a complex chart, which just isn't performant in SL today due to the need to create (in some cases) tens of thousands of shape objects.
I would REALLY like to see more compatibility with WPF (and more parity with Flex/Flash) in this area, and SL3 would be a great place to do it. Odds are that the rendering engine in SL has such an API, but just doesn't expose it...
One unanswered SL 3.0 question, one that, to me, is critical for line of business apps, is support for a lower level drawing object/API (e.g. DrawingVisual in WPF). Even marginally complex business charts with more than a few hundred data points exhibit
poor performance due to the need to instantiate so many shape objects (lines, rectangles, etc.). Any plans to add these capabilities in SL 3.0?