Hard drive space. I have my system and apps installed on a 60GB OWC SDD. Roughly speaking (again, best guess based on my recollection, but I can confirm against the CCC clone I have at home in case of emergency), I had about 30 GB free under Lion. I now have about 40 GB free, with part of the used space taken up by the ML upgrade downloader so I can burn an emergency USB drive at some point.
I don't think I've ever seen an OS upgrade that reduces the space used...