WordNet sense 1 (lacking taste or flavor or tang):

flat, flavorless, insipid, vapid, flavourless, savorless, savourless

WordNet sense 2 (not stimulating):

unexciting, unstimulating

WordNet sense 3 (smoothly agreeable and courteous with a degree of sophistication):

politic, smooth, suave

WordNet sense 4 (skilled in dealing with sensitive matters or people):

diplomatical