JennyC, I'm from South London originally myself, moved up here in 2005 to be near husband's family and pretty happy here.
Good points (personal opinion only!)
Some decent schools if you move to the Lordship area, or Willian/the south side of Hitchin, especially the primaries. Secondaries ok, pretty much in keeping with what you'd expect from the Shires! Depends though on what you're used to in your area of London (have a look at the Ofsted website) and what you think of as a "good" school.
Feels safe, pleasant enough
Nice housing stock, some pretty fancy stuff especially to the south. Unaffordable to most but nice to gawp at
Close to supermarkets (Morrison's, Sainsbury's). Massive Tesco in nearby Baldock.
It's an ok commute into London too (expensive tho). Get a fast train and you'll be in London in half an hour.
Nice, non-chain cinema
Bad points:
I find the lack of decent shops in a town of 30,000 people depressing, but I grew up with access to Croydon and Kingston etc. I think they are starting to redevelop though.
Town centre dead at night, you're not going to find many (any??) decent restaurants (go to Hitchin though, which is much better. Or even Baldock)
Can't really comment on things to do with kids though, am expecting my first one so am kind of ignorant. Hope someone else can fill in the gaps for you. If you've got any specific questions please just ask tho, happy to try and help!