I would say the biggest mistake with the 2nd runway was not building a taxiway all the way to the far western end; thus requiring any aircraft landing on 23L, or departing on 05R to back track on the runway.
I think there was a plan to do this in the future.
In the original planning, there was a plan to have a parallel taxiway for the second runway. However it was dropped because
1) The way the runways are normally used because of the spacing between them - landing on 05R or departing on 23L doesn't require one. A parallel taxiway is only required when landing 23L and departing 05R which doesn't happen often (although all this summer whilst R1 is fixed but at night-time only when traffic is generally lighter)
2) A parallel taxiway would require much extra land take, a much longer tunnel over the river and A538 and the cost could not be justified with so little regular use and no capacity gain in normal operations.
3) Bearing in mind the above and the planning climate, it was decided not to proceed with it.
Who knows whether it will ever be resurrected - I guess not in the foreseeable future anyway
Suzeman