Huge museum, rich collection, well thought recesse...
Huge museum, rich collection, well thought recessed lighting - no flickering light on the paintings. The last one is really important - I've been to too many museums with great collections and terrible lighting (Uffizi comes to mind, where they have beautiful Botticelli paintings under glass placed right in front of windows - can barely see what's there)
Paintings in the National Gallery are well placed, in logical order - the museum is really easy to navigate.
I spent 3 hours there and could easily spend twice as much - so big is the collection. Holbein, Raphael, Van Dyck, Renoir - every big European painter is there and well represented.
On top of it, the museum is open 7 days a week and the entrance is free.
IMHO, the National Gallery is must-see when in London.