I tend to worry that some of the proposals on Area 51 have the potential to dilute the Stack Overflow audience. For instance:
These are just a few. As one comment stated, these seem more like sub-sets of Stack Overflow.
Note: Re-read the Web Apps proposal...I can see where it is different.
To further distinguish this question from Area51 proposals that overlap StackOverflow : I am concerned that SO will become a group of C# developers who are concerned only with SqlServer in C#, building forms in C#, regex in C#. I like coming to SO for my programming questions. If in a few months, I somehow start using jQuery, or Drupal...I still want to come to SO to get my question(s) answered.