Even though the terminals were completed in 1992, PIT is showing a little age already. I think this is partly because it was built for lots more passenger traffic than it currently sees, so with vast concourses and empty spaces everywhere, you start to notice things like the terminals' age a bit more.
It was once a major hub for USAir, although that airline never really found firm financial footing and scaled back dramatically after 2001. The expansion of airlines like Southwest, JetBlue, and even Allegiant has helped offset the decline in passenger traffic somewhat.
These days, PIT serves a variety of domestic and international airlines and generally manages to get you to many places in the eastern U.S. despite the fact that it's no longer a hub. To try to boost traffic at the airside "air mall," they've even opened things up to non-passengers on weekdays. Overall, it still gets the job done - just a bit quietly.