You can tell by watching this episode that it is one of the earliest episodes of Friends. The actors don't seem too used to their characters yet, and the writers also don't seem very used to the show. That's not too much of a problem though. We can't expect any more from them at the second episode.
What it really was that made this episode so unique was its format. The second half of the episode had barely anyone in except Ross and Rachel, and it alternated between their scenes in a fashion that I must have seen in over a dozen other shows. Yes, it did its job, but I didn't like it very much. It was interesting to see Carol, Susan and Barry for the first time. After watching the Pilot I wouldn't have expected that they'd ever actually be in the show.
Carol being pregnant was a very surprising incident, and it was good since it gave season 1 one of its biggest storylines, not to mention the fact that it hugely affects the show as a whole, obviously.
Monica's plot in the first half of the episode was my favorite, and gave us a LOT of character development, and we met Jack and Judy, who, like Carol and Susan, would appear in many episodes in the future. I found the Ross, Carol and Susan scenes too, where they argued about the baby, something that we get to see more of throughout the season fortunately.
Overall, a good episode.