I am currently in Paris in the 20th with two kids and a student DH. The area doesn't have a great rep, but I love the part I am in: walking distance to Bastille, Nation, Bois des Vincennes, easy access to both Gare du Nord and Gare de Lyon, cheap markets and fancy boutiques side by side. I agree with beachcomber that sometimes you can feel like you've gone back in time here in France....cheques, having to pay for online delivery for everything, even collect in store, much more bureaucracy done in person or by mail. We have been here for two years and there is always at least one piece of paperwork we are waiting for or some office we need to go to.
Every day things are much more expensive: food, clothes, going out, everything. I often order things online from the UK and have them delivered here.
I think I would adore living in Paris if I were part of a young couple. I would try to live around Bastille in the 11th arrondissement or in the 5th near the universities. Around montparnasse or in the 6th would also be great, as would around st paul in the 4th. I think around bastille would be ideal though for the best transport links and I think prices would be a bit cheaper. Flats are small, but everyone is used to it and it is easy to find furniture to fit.
As far as meeting people goes, everyone I have met is desperate to speak and learn more English, so I don't think you would have trouble meeting people and building a network though it does take a big effort at first. There are lots of cafes that do language exchanges and social enterprises so you would find things to do.
I would set aside some money for an intensive French course if you want to brush up. I have heard that there are also language labs in the library near Centre Pompidou and TV has helped me too.
Having a baby here is brilliant, though again I would buy the baby stuff from the UK except for a few pieces of extra cute petit bateau/bonpoint stuff. The birth experience really depends on which hospital you register with and usually spaces are booked up quickly, sometimes before you are six weeks pregnant. Make sure you call the maternity hospital you want as soon as you get a positive test! Likewise, you can register for childcare from the sixth month of pregnancy and places fill up fast. I have really benefitted from the extra things they offer here like dietician appointments and help with your pelvic floor muscles after the birth.
I have moved around a lot and view living in a new place as an experience worth having even if the new place isn't as easy to live in as the old one. I really value picking up a new language and viewing the world in a different way, so I would say if you are up for a change and have the energy for the initial period of settling in, go for it.