Unfortunately, you would not save any HDD space at all since Windows Features
only turns off or on the Windows 7 feature and does not unistall it.
Yes, there could be issues to turning off a feature if you or Windows 7 is using it (ex: .Net Framework), and having stuff no longer work all of a sudden in Windows 7. I would advise that you only turn off the ones you know you do not need, and only do them one at a time and test inbetween to see if you have any problems with it turned off.
To be safe, create a restore point
before doing this so that you can do a system restore
back to that restore point if needed.
Hope this helps,