hippietrail has already done a good job of adding descriptions and excerpts to tag wiki and there are many still in queue (which I regretfully can't approve because I have a daily approval limit :(). His edits are consistent and most tag wiki excerpts and descriptions begin with "Questions about X" where X is the description of whatever the tag refers to. It seems to me like a good enough scheme, but most edits by other people don't follow it, and I think it's better if we have a standard.
Looking briefly at other SE sites, it seems to me like the situation is different in each site. I think actually I prefer Stack Overflow's approach, which doesn't use introducing phrases like "Questions about..." or "This tag marks topics about..." for tags that simply refer to terms such as C++, Java, networking, etc. It's obvious that the users going to the tag wiki for these tags want to know what these things actually are, and they don't need the introducing phrase, since it's kinda obvious that the
C++ tag refers to questions about whatever C++ is and the same things goes to the
Now, I don't think adding these introductions would be so bad, but if we do, I think we should at least be consistent about it.