The South Downs National Park decision process has been held up by a legal challenge on the meaning of "natural beauty". Defra has details of the challenge on their web site. Essentially the court found that Hinton Admiral Park did not qualify as possessing "natural" beauty, so it could not be included in the New Forest National Park.
Defra appealed, lost, and changed the legislation, so that farmland, managed woodland, parks and other areas affected by human intervention could be included in National Parks to preserve their "Natural Beauty".
So, Defra have now restarted the decision process. They've begun a round of consultation, and may even reopen the enquiry.
I support the National Park in principle, but I'm not sure about the precise boundaries. That doesn't mean I oppose any of the proposed boundaries, it just means I've not considered the issues.
East Sussex County Council seems to have opposed the National Park on the grounds that there's no proof that the Park Authority will manage the park with better financial efficiency than current arrangements. Frankly, I'd have been embarrassed to be associated with such an argument. The point of the Park is surely not simply to improve financial efficiency, and this really isn't a logical reason to oppose it - heck, they're not even claiming that money will be wasted, just that there might not be any savings!
West Sussex County Council seems to have made the same objection, and the rather more principled objection that planning will be taken out of the hands of the elected authorities. I have more sympathy for that argument, and that might be a good reason for keeping Lewes, say, out of the Park boundaries. However, I believe that the Park can delegate some of its planning role, and that any problem here would lie in the implementation, and not be a reason for objecting in principle to the Park.