WordNet sense 1 (lacking wit or imagination):

pedestrian, prosaic

WordNet sense 2 (inducing mental lethargy):

soporific

WordNet sense 3 (lacking interest or significance):

insipid, jejune

WordNet sense 4 (so lacking in interest as to cause mental weariness):

dull, irksome, tedious, tiresome

WordNet sense 5 (organized as or forming an institution):

institutional