I live in Diss! Personally, I really like it. It's small enough to be friendly but big enough to have shops/pubs/restaurants/Corn Hall for theatre and v limited cinema etc. There are lots of independent shops, not so many chains. We're also on the Norwich to London trainline which is very handy, 20/25 mins to Norwich and Ipswich, 90 mins to Liverpool Street. My dd's go to an Outstanding school, but I dont' have any experience of the High School yet.
You do sometimes hear people say negative things about Diss which baffles me, I can only assume they're comparing us to somewhere poncy like Woodbridge or something.
I don't know Downham Market at all and haven't done much more than drive through Long Stratton so can't really comment on those places.
I live in Suffolk. (apparently where we live is poncey)like Diss but people are rude about it,partly because it is in the middle of nowhere and not easy to get anywhere.Know quite a few people who moved when their children got to secondary age.Have you thought about Wymondham ,similar size ,easy reach for Norwich and fantastic schools .
Not easy to get anywhere? Can't say that's my experience. Norfolk as a whole lacks motorways and dual carriageways (some would say that's part of it's charm!) so we're no better or worse than anywhere else but we do have the advantage of being on the trainline.
One of the advantages of Diss is that you can commute from here to a number of places, you can get to Norwich, Ipswich, Bury St Edmunds and Great Yarmouth in less than an hour, Cambridge in an hour and 15 mins I know a few people who even commute to London every day.