As for the why of iOS5 draining your battery much faster than the previous OS, this Fastmail blog article might give some specific clues (Fastmail is an IMAP email provider):

Unfortunately, there’s a new and recent problem that has now appeared.

iOS 5 now uses long lived persistent IMAP connections (apparently previous versions only used short lived connections). The problem is that our ping packets every 4 minutes mean that the device (iPhone/iPad/iPod) is "woken up" every 4 minutes as well. This means the device never goes into a deeper sleep mode, which causes significantly more battery drain when you setup a connection to the Fastmail IMAP server on iOS 5 devices.

Given the rapid increase in use of mobile devices like iPhones, and the big difference in battery life it can apparently cause, this is a significant issue.

As for the technical why of iOS5 draining your battery much faster than the previous OS, this Fastmail blog article might give some specific clues (Fastmail is an email provider, and the issue described applies to the IMAP email protocol, but it might be hinting about a more general change in iOS):

Unfortunately, there’s a new and recent problem that has now appeared.

iOS 5 now uses long lived persistent IMAP connections (apparently previous versions only used short lived connections). The problem is that our ping packets every 4 minutes mean that the device (iPhone/iPad/iPod) is "woken up" every 4 minutes as well. This means the device never goes into a deeper sleep mode, which causes significantly more battery drain when you setup a connection to the Fastmail IMAP server on iOS 5 devices.

Given the rapid increase in use of mobile devices like iPhones, and the big difference in battery life it can apparently cause, this is a significant issue.