Just that really.
2 of my old Halifax accounts became dormant - my own fault I just let it slip and kept thinking I'd sort it out. They didn't have enormous sums of money but I suppose I always thought of them as a bit of an emergency fallback fund. It took me ages to get them reinstated and I had to go in branch with ID etc. It was a real pain and while they're dormant you can't use them.
They told me just to set up a small standing order to keep them open even if its only a fiver or tenner every 2-3 months - that way they're showing transactions and won't become dormant.