well,
As Aaron Copland, explains in his book :" what to listen for in music" , melody is not the only thing, AND not every composer is equally gifted as a melodist. The example he gives is: how stravinsky's melodies are comparitively unimportant, but prokofiev on the other hand, works a seemingly inexhaustible well of melodies. yet few people would say that prokofiev is the greater composer ( in my opinion , they are both out of this world, and i dont prefer one to the other , even though ,i love prokofievs piano works more).
Melody in debussy's pieces are extremely fragmentary, especially in his later pieces ( i think Pierre Boulez even used the word Musical Pointillism , to describe his music), and therefore , the preludes have really short melodies, its not that it doenst have a melody, its just that its short enough that you dont realise it , before another melody pops up.
whatever it is, Debussy is one of the greatest musical geniuses that ever existed. He rocks!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! i love ALL of his music.
but thats my opinion .
and oh ya , i love all his preludes. but the first time i heard them , i didnt like all of them, infact out of the 24 preludes, i only liked 3 or 4 to begin with, and i was quite dissapointed with the rest , to begin with, but when i heard those pieces again, it started making sense, and now i am obsessed with not only all of his preludes but all of his music. so , atleast try listening a couple of times more. you dont have to keep listening 20 times , cuz if you dont like it , you dont. but atleast give it a fair chance, listen to it a couple of times more. things might start making more sense.