Most Natural History museums I've been to are the standard old school dioramas and stuffed animals. While the Carnegie has these things, it offers so much more.
The dinosaur exhibit has to be one of the best out there. It shows them in what would have likely been their natural habitat and groups them according to the period they were from. They also have interesting information on how they came to acquire such a great collection. There are guided tours daily that are free with admission and I highly recommend it!
The gem and mineral collection is top notch. I especially enjoyed that there was a particular amount of attention paid to those gems and minerals that can be found in Western PA.
The Egyptian exhibit is well done presenting great info and insights into the life of ancient Egypt and the Arctic exhibit really provides a connection to the day to day life of the Innuit people.
There is a lot other great exhibits as well.
Overall, the museum is well worth a visit and I look forward to returning soon.