I do find it interesting that two of the biggest critics are from WGC and that the loss of the fast from WGC to Finsbury Park does appear to be so 'important'. WGC today has FOUR decent well timed services into and out of KGX that provide much greater flexibility than ever before. Try changing at Finsbury Park for services into the core, I have done 3/4 days this week and been at work 10 minutes earlier than I was last week. The WGC critics are a little too NIMBY about this. When this settles down it will be a wonderful new service, yes it's going to be frustrating at times but look at what's running and arrange your travel accordingly for a few weeks.
I suspect the issue at Welwyn GC is more to do with capacity than two extra stops. In the morning peak today's 4tph service is replaced by ... 4tph, except the seating capacity has been reduced from the days when these services were formed of 2x317, and the journey is slower on top of this. Perhaps if Welwyn GC saw some 12-car services in the morning peak then people would be happier. Likewise these services will not be running through the core for another 18 months thanks to Thameslink's failure to deliver its initial promises - so no trade-off benefit here in the form of through journey opportunities.
Of course a lot depends on how busy the trains will be when they arrive at Welwyn GC. That is quite an unknown, and still is until things settle down.
I'd say some of the places further out have a lot more to complain about. The Peterborough line is seeing a nosedive in reliability albeit some extra peak capacity. Letchworth and Hitchin are losing their non-stop services to/from London, as well as not seeing a massive increase in high-peak capacity -- especially until the second Cambridge/Brighton service materialises. Everywhere is already seeing a nosedive in reliability, which will be particularly felt on down journeys, both from London and between local stations. There are also plenty of 8-car services still floating about, both in the peaks and the shoulder peaks - for example I'm hearing that the Royston/King's Cross services have been horrendously overcrowded this week.
What should be a roughly fifteen minute journey from Stevenage to Biggleswade now stands a much bigger chance of taking much longer should the train be delayed somewhere south of London - even more so if stops are then omitted to make up which is what is already being seen.
(PS - am I the only person who absolutely detests the term NIMBY?)