Dropbox is great, and it's one of the few companies I don't mind pimping out. However, even I have my limits, and "the best 30 comments on this post explaining why Dropbox is awesome will be given 5GB, of extra storage on their account" crosses the line. It's just ego stroking and obvious advertising at that point.
If you must, why not reward people with the best ideas to improve dropbox or who have unique ways of leveraging it? I'll give you $5 to tell me how great I am...seriously?
The best comment was about interesting ways to use dropbox. Some even I hadn't heard of. The second comment thread was recursive actually, showing how awesome reddit can be.
These things are more fun than serious. Throwing around GBs to people that like Dropbox is really common (see Dropquest, http://dropbox.com/eduhttp://dropbox.com/free ). I wouldn't read too much into it here; folks just thought it would be nice and fun.
btw I work for Dropbox. I ran that reddit mini campaign.
If you must, why not reward people with the best ideas to improve dropbox or who have unique ways of leveraging it? I'll give you $5 to tell me how great I am...seriously?