I'm going to be really boring and point out that the writers have made a conscious decision to vary the style of episode. "All things to all men" (and women, and children, and animals, and small furry creatures from Alpha Centuri, indeed all sentient, carbon based life forms).
There will be episodes aimed directly at children. Judging from the trailer, probably the next episode. (Note, scenes involving scared kids. Yes kids, you could be next!!). Episodes involving old enemies such as Daleks and Cybermen. Episodes involving new enemies, (dunno who, haven't seen them yet). Episodes which are "How scary can we make this and still get broadcast" and, yes, episodes which are just plain silly, such as Robin Hood. (How many scenes were stolen from various Robin Hood films?).
It's not the first time there have been silly episodes. Remember "The Unicorn and the Wasp", with David Tennant. The villain was a vicar who turns into a wasp! Implausible even by Dr Who standards.
I suspect little girls were delighted with this episode. Yes, Robin Hood is real, he acts just the way he should, Maid Marion is not wet and fawning, and Clara (i.e. the surrogate
you, if you are a twelve year old girl) gets to kiss Robin Hood!!
I really enjoyed the episode. Of course it was silly. Part of the fun is the Doctor's deadpan jokes. (Actually, a lot of the fun is the Doctor's deadpan jokes. The style of humour appeals to me greatly). Part of the fun is spotting scenes from films (I think this one had part of an Errol Flynn film). Part of the fun is spotting references to old Dr Who episodes. Anyone remember which episode had the Miniscope?
Sit back and enjoy. With this many hours of TV, you have to expect somewhat variable quality, but on average, it's still one of the best programs ever made.
Well, that's my opinion anyway, and I'm sticking to it. So There!