Rachel Dratch was supposed to play Jenna Maroney, but NBC decided she wasn't hot enough. She was all over the first season of 30 Rock as a consolation.
I 100% feel they did Dratch dirty, but at the same time, I really enjoyed Krakowski as Jenna and cannot imagine anyone else playing her. Dratch is also a fantastic character actress, and I really enjoyed all the various one-off roles she played through the show's run.