SlideShare have added some new email+web based options for uploading PDF and PPTs that makes the service all the more attractive.
For instance, if you want to move your presentations from Scribd to SlideShare, just type their URLs here and SlideShare will automatically make a copy in your SlideShare account.
[Like Scribd, Slideshare also lets you upload Presentations anonymously.]
Another addition is the email based uploader (a similar feature exists in Google Docs). You just have to send the presentations as email attachments to a specified address for uploading them onto SlideShare.
Wishlist - Now that Slideshare has bulk upload, they may consider adding another feature that allows members to download multiple presentations in a batch.