Many moons ago we used to have an acronym for customers who couldn't be sat by self-help emergency exits called CODPIES;
C - Children
O - Obese passengers
D - Deportees and Disabilities
P - Pregnant and Prisoners
I - Infants on laps
E - Elderly
S - Sick
On the Airbus 320 family, the self-help exits as described would be the overwing exits as there isn't a crew member sat there.
Front row bulkhead seats where there is a galley or a fixed bulkhead between the seat row and the door and where there is a crew member sat next to the exit are not classed as an emergency exit row. Obviously with Row 21 on the Midhaul A321 there isn't a bulkhead or galley between the row and the seats, but there is a crew member who is sat at Doors 3 and would be responsible for those doors. In an unplanned emergency, you don't want to have anyone who could cause an obstruction to the crew sat in that row, however it's open for debate as to how much a sling would impede an evacuation when there is a crew member sat by that door.
Having just come back from CAI in the last few days on the Midhaul Airbus, I have to say that the only issue has got to be that there is less hand baggage stowage than on the widebody aircraft, which is purely down to British Airways' very generous hand baggage allowance. Other than that, they are bright and airy, brand new seats, the brand new Thales state-of-the-art IFE and lovely steam ovens and once customers got over the initial shock of not being on a Jumbo, the feedback was overwhelmingly positive in both CW and WT.