I found this bash script for downloading all the WWDC 2014 videos in HD.
curl https://developer.apple.com/videos/wwdc/2014/ \
| grep -iIoh 'http.*._hd_.*dl=1">HD' \
| sed -e 's/\?dl=1">HD//g' \
| xargs -n1 curl --remote-name
If you'd rather have SD, then
curl https://developer.apple.com/videos/wwdc/2014/ \
| grep -iIoh 'http.*._hd_.*dl=1">SD' \
| sed -e 's/\?dl=1">HD//g' \
| xargs -n1 curl --remote-name
should do the job.
I went for HD as Apple seems to be using better compression this year, and the 2014 HD videos are smaller than the 2013 SD versions.