Q:

Will these proposals have a potential to dilute what Stack Overflow has going?

I tend to worry that some of the proposals on Area 51 have the potential to dilute the Stack Overflow audience. For instance:

Web Applications
Drupal Answers
Regex
IDEs

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.

A:

I wouldn't worry just yet. A Proposal is not a site. There hundreds of people scouring Area 51 just looking for something to do. But, there's a lot more to getting a site created than getting past the straw polls of the Definition Phase.
My guess is that the good-but-not-needed proposals will languish into non-existence once real people are asked to commit real time and real resources into a site idea that is already covered elsewhere. It would be unfortunate if a bunch of mediocre sites are created out of boredom for something to do in Area 51.
Your question applies not only to Stack Overflow, but all sites on the network. There are still some fundamental issues regarding the best way of scoping out each site:

Create smaller, niche sites versus creating mega sites with tagging
Selecting site topics that optimize the network versus letting every site stand on their own
Slicing up the Q&A space into neat little segments versus letting overlapping circles form around expert groups
Letting sub-sites splinter off from sites that already cover the subject

It's easy enough to pick out a few, individual proposals that just seem somehow... infringing. Its an entirely different matter to come up with a general policy to rule them all.

My best guidance is to vote with your commitment to the site.

If the site isn't going to make the Internet a seriously better place for you, find something better to do. You don't have to jump into every site proposal that you could ever possibly have interest. If you don't want a site created, don't follow it. If you see a site as redundant—even if you would use it anyway—don't be afraid to ignore it. It's not as if you are banned from using the site if it gets created without you.
Direct your energy and enthusiasm towards those few sites  that you truly, really want. Ignore the rest. It's the only way to separating great site ideas from the cruft—by measuring the commitment you put into it.

