Hi
The main issue is interference with Primary Radar.
Wind Turbines can create clutter because the blade tips are travelling at speeds of around 100 knots which mean that they get through the filters that remove static objects.
Other concerns relate to physical obstructions, potential interference to SSR and a few other things like Nav Aids and ILS.
In general concerns can be overstated - I have to declare a bias - I work for a consultancy that helps wind farm developers overcome these issues.
Hope this Answers your question.
You can find out more at
www.pagerpower.com and
www.bwea.com/aviation
Regards
Windfarmer