I believe that Chopin's music is just as strong as Bach's. He was just as intelectual about composition, only in a different way. He created his own forms, possibly because his own sense of symmetry forced him to do so. Also, the modulations were why he is remembered, and why he is known as one of the great composers. Unconventional modulations aren't the mark of a weak composer, rather, they are the mark of a strong one, as he isn't afraid to go against the convention of the last few centuries. For example, Schoenberg is still known for the twelve-tone method, which doesn't use conventional scales, but is still one of the most intellectual ways of composing ever.
And I don't believe Chopin is always sad... Just look at his F# major Impromptu, any Waltz in a major key, a few of the Nocturnes, and the prime example: the Polonaise in A flat major, op. 53.
I would agree utterly and completely. Look at his Polonaises, heroic, majestic, and patriotic. Scherzi, dark and sometimes humourous. Waltzes, refined, graceful, elegant. I could go on...Barcarolle, Tarantelle...Preludes...seriously, how could one even dare compare Chopin to something remotely "weak"? More like a spectrum of wonderful emotions, brought about by colourful harmonies and incredibly idiomatic writing.