I still don't know where all this DUNE stuff is going. Spice? Okay. Maybe. But the word spice and spices' collective value as a traded commodity goes back many centuries here on Earth, so it may have had nothing to do with Frank Herbert. At all.
Psst. Hey. Do you want to buy some really good oregano?
Remember, Lucas immersed himself in history and mythology and I don't think we can just say that Han dumping a spice shipment meant for Jabba is any proof that DUNE is directly referenced. If it was, fine. Cool. But....where's the evidence, though? And does it matter?
Oh, it's probably an allusion
or, if you prefer, a reference.
Given all the other apparent shout-outs, the odds that something so prominent in Dune
isn't being shouted-out to are kinda low.
As to what all the nods are, you've got: a desert planet, Dune Sea is a pretty clear wink, the Tusken Raiders and their desert wear that collects moisture, that giant snakelike skeleton in the desert as already mentioned, the word spice,
and maybe a couple of other minor tidbits like seeker droids (cf hunter-seekers).
You put it all together, and it's like, OK this is crafted to appeal to people who've read Dune.
That's the bottom line.
You could say the same thing about it being crafted to appeal to people who like Star Trek.
You've deflectors, tractor beams, Federations (in the PT), etc.
Or Flash Gordon.
You've got scrolling prologues in perspective, space Emperors, maidens in space in peril, etc.
Sorry to those who think otherwise, but these aren't rip-offs.
Here's a nice little bit from http://wiki.answers.com/Q/What_made_..._a_popular_hit
Star Wars is an intentional pastiche of nostalgiac references to 1940s and 1950s pop culture (especially old movie serials) and especially Flash Gordon, as well as an homage to the cinema of American westerns, and the films of Kurosawa, John Ford, etc. , among others. This self-conscious nostalgic pastiche in Star Wars (story lines, themes, etc.) is perhaps misunderstood by the minority who saw Star Wars as not citational but plagiaristic. Clearly, however, its nostalgic elements resonated with older audiences who recognized the themes and formulas that George Lucas was playing with, even as younger audiences were exposed to these often for the first time. Star Wars in many ways started the trend toward "retro" in movies--not merely remaking old movies, but mixing and matching retro elements into something new and different.