When rating Shakespeare, I always rate his works as compared to other Shakespearean works; otherwise, the consistently high marks wouldn't be very informative. For instance, if this were to be rated against the general run of literature extant, it would certainly rate five stars. Even by the standard I'm using, it's close.
Like "Hamlet", this is a tragedy that still manages to have some very funny lines; as in "Hamlet", this is generally due to characters either pretending to be crazy, or truly being crazy, so it's something of a dark humor, but humorous it still is. Lear's jester has some great lines doing what only a jester could get away with (and what the reader wants to do): telling the King that he's an idiot when he's done something ignorant beyond belief. Edgar, son of Gloucester, banished by his father for supposed treason, plays the part of a mad beggar to save his life, and when Lear, honestly crazy from grief, meets up with him, their conversations rival anything in Hamlet for manic nonsense that still manages to make a certain warped and poigniant sense.
It's a shame that the language has changed so much since Shakespeare's time, so that the masses are unable to enjoy and appreciate his wit; his plays were not written to be enjoyed only by the literati; they were intended to entertain and, yes, enlighten the masses as well as the educated; his plots seem to be right in line with either modern romantic comedies (in his comedies) or modern soap operas (in his tragedies). Modern audiences would love him, if only they could understand him; unfortunately, when one "modernizes" the language in a Shakespearean play, what one is left with is no longer Shakespeare, but simply a modern adaptation. Which, if done well, is not without value, but is still far short of the original.