Right - views on Watford in terms of your criteria!
More affordable: yes, it's more affordable than other parts of the Hertfordshire commuter belt like St Albans, Harpenden and Berkhamsted. It's less pretty than those places though (which explains why it's more affordable), but actually plenty of nice bits of Watford (think North Watford, Tudor estate, Cassiobury estate, Nascot Wood). You could also consider Rickmansworth?
The choice is basically to pay c.£100k premium on house prices and live somewhere pretty like Harpenden or St Albans, or save the £100k, live in decent Watford, but travel the short distance to St Albans/ Harpenden anyway if you want nice restaurants and expensive independent shops!
Nice high street: Watford has a great town centre, with the Intu shopping centre (a big John Lewis, Debenhams and all the usual chain shops like H&M, M&S etc), a brand new cinema right in town, lots of restaurants (mostly chains). But it's not pretty pretty, and there aren't loads of chic independent shops and cafes.
Things to do locally: Watford has two public swimming pools, two theatres (the Colosseum, Palace Theatre), a fantastic huge park (Cassiobury) with two nice cafes in the park and big splash park in summer for children, a bowling alley, various soft play places, other parks and playgrounds. Also masses of day trips v close (RAF Hendon museum for the airplanes, Natural History Museum in Tring, Stockwood Discovery Centre in Luton, the whole of London, etc etc etc).
Somewhere with a rural feel: nope. But you could live in Abbots Langley, Kings Langley or Bushey, all basically outskirts of Watford, which feel more rural. And from where we live, you can be in the countryside in 10 mins drive - there are some lovely bits of countryside. Look up Whippendell Woods, but lots more than that.
Commute an hour door-to-door to Liverpool Street: yes, that's definitely do-able from Watford Junction or Bushey.
Excellent state secondary schools: yes, definitely. You need the South West Herts Consortium, which is a group of semi-selective state schools, where you can get in on an academic, music or sports test. There's also the usual distance criteria, so it's possible to live close enough to get in on that. For the most competitive schools in that consortium (Parmiter's, then Watford Boys' Grammar and Watford Girls' Grammar, and St Clement Danes), you have to live very very close. But Queens, in the consortium, has a lower entry bar, and is perfectly good. Because the consortium schools tend to cream off the most academic kids, the other state schools in Watford tend to have much lower academic results.
Bear in mind that the Boys' and Girls' Grammar schools are not actually 100% grammar - they only allocate a certain percentage on the academic test (I think 30%) - the other places are the music test, distance, siblings etc.
Safe and friendly: so @JoJoSM2 suggests Watford isn't that safe. The comparison website says Watford's overall crime rate is lower than Oxford, Reading, Northampton or Southampton, and higher than Milton Keynes, Hillingdon and Cheltenham. Personally I feel perfectly safe, and I don't know people who have experienced lots of crime here. According to that comparison site, the burglary rate is actually a lot lower than comparable areas, and vehicle crime is slightly below average compared to comparable areas.
Friendly - my DC have made very firm friends; most families live very close to the primary school my younger one goes to. My older one is at secondary (happy to discuss over PM which school and what we think of it!), and has some lovely friends through that too.
Hope that helps and happy to answer any more specific Qs!