I am a simple surgeon and know nothing about airport/airline management, these are just my thoughts.
I studied at Bristol university and then worked in Plymouth for 6 years and so Bristol was the closest reasonably large airport and I used it a lot. Since moving to Birmingham I still use Bristol often if I find a flight at a better price/more convenient time. I certainly love Bristol as a city and like am quite fond of the airport.
However, I think it has been a victim of it's own success. When the new terminal opened it was spacious, airy but also empty and lacking in facilities. It now has much better facilities in terms of shopping/catering but nowhere near sufficient room for current passenger numbers let alone the predicted increase.
The terminal needs expansion and fast - not just adding little bits here and there as they've done the last few years, but major expansion to at least twice its size.
I have never flown the continental route to New York, but was planning to next year. Obviously I won't be able to now. However, I can see that the major users (ie business people) as well as many leisure users would be put off by the lack of facilities at the airport. Particularly for potential passengers that live on the London side of Bristol, why would you go there and spend a couple of hours in an overcrowded terminal with relatively few facilities and few seats, where you have to join a queue for the gate that snakes the length of the building rather than a dedicated gate waiting area, and then catch a bus or walk outside through what looks like a partially completed metal air raid shelter. Why wouldn't you go to a larger London airport where you have a good choice of shops and eateries, decent seating, and airbridges right up to the aircraft.
Will the Western walkway really improve things? I understand they cannot really add additional facilities such as seating areas due to planning. Will there be conveyers to ease the long walk? Are there air bridges? There certainly should be both of these, but I suspect it will be done on the cheap without either.
I would consider Birmingham airport as the sort of airport Bristol should aspire to. I personally think Bristol needs a runway extension to handle larger aircraft, and I'm amazed that this isn't in the major expansion plan (as far as I know - I may be wrong). On a longer flight, such as to New York, I would personally rather fly on a bigger aircraft than the 757 used from Bristol.
I think Bristol should also be looking at route via Dubai as this is a hugely growing area. My wife is from Malaysia, and we fly regularly via Dubai with Emirates. I would love to see something like an Emirates 777-300ER flying from a much improved Bristol in the future and, as I plan to move back to the Southwest, would certainly use it.
I just hope this expansion goes ahead.