I'd have to say that Square Cafe would be my top pick so far for brunch in Pittsburgh.
Its a pretty standard brunch place with cute decor and crowded saturday mornings, with people meeting up to catch up from the week, but it also has a great menu selection, lots of seating, and the portions are on the larger side.
Only complaint I have is that coffee is a little pricey and I know some people like the little mugs they use, but I felt like mine was a little dirty :(.
So far I've only tried the breakfast foods, and all were quite tasty. Service was also pretty good, even though it's always crowded at Square Cafe. They also have a large section of seating outdoor, which I love!