Lack of a good free UML designer for .NET, or just in general
I have found that .NET really lacks in this space, needless to say I've been scouring the web for a decent free UML designer.
Annoyingly most of the free UML tools have heavy restrictions on there use (VP, MagicDraw), oh and Visio is hopeless for complex diagrams.
NetBeans comes with an awesome UML designer built in and supports the 8 core diagram types.
The discouraging thing about the Visual Studio space is there hasn't been word of class diagrams, or the modelling space in general since MS announced support for C++ class diagrams in March 2007.
With all the designers that are in VS now for literally anything you would sort of think that this UML stuff would of crept in somewhere along the line.
Maybe in the next VS release there will be something on this???