Is installing a HDD instead of SuperDrive safe? It's pretty popular to replace ones optical drive* on the MacBook Pro with a HDD, but how serious are the risks?
*In most cases, you'd do this when installing a third-party SSD, but I'd like to leave any of those benefits out of this and isolate matters of the HDD and Superdrive.
I mean things like:

Overheating
Performance issues
Battery life
Warranty status
Weight increase
Upsetting Steve



Update: The above questions have been addressed in this question, but there is a more serious issue — potentially, a missing sudden motion shock sensor in the optical drive.
Overheating shouldn't be an issue. The SuperDrive probably runs
hotter (when it's in use) since it has moving parts
An SSD would most likely improve performance (especially if you use it as your boot drive and place for applications)
Don't know about battery life, but I imagine if you you use this as your boot drive, and the stock hard disk as your "slave", you might actually see an improvement (again, fewer 'moving parts')
This will void your warranty, but if you're careful you might be able to get in and out without leaving evidence of tampering. Consult iFixit's guides for tips
I can't imagine there would be a noticeable weight difference. 
Steve is dead, unless you're talking about Steve Zahn. Steve Zahn doesn't care what you do with your computer.