SPDY requires SSL and SSL for all intents and purposes doesn't work with name based virtual hosting (it doesn't for all versions of IE under XP and on Android < 4) due to the lack of SNI.
Aside of a noticable hit in performance for the encryption work (some say insignificant which is probably true if you are google), enabling SSL is a considerable additional administrative hurdle both because of getting certificates (and keeping them fresh) and because of the requirement of needing one IP per domain.
The latter problem is going to get worse as more sites want to go SSL and we're already out of unassigned IP addresses.
If only we could have widely supported SNI. Then we'd back at only the performance issue (which is going to get less and less noticeable as our hardware gets better) and the administrative overhead for the certificate.
Aside of a noticable hit in performance for the encryption work (some say insignificant which is probably true if you are google), enabling SSL is a considerable additional administrative hurdle both because of getting certificates (and keeping them fresh) and because of the requirement of needing one IP per domain.
The latter problem is going to get worse as more sites want to go SSL and we're already out of unassigned IP addresses.
If only we could have widely supported SNI. Then we'd back at only the performance issue (which is going to get less and less noticeable as our hardware gets better) and the administrative overhead for the certificate.