While we shut down our system, all the user as well as kernel threads are killed and the devices powered down. So while booting up, you extract the kernel and then again initialize all the drivers and other services.

Since there are not really any changes in the running kernel, is it ok to kill off all the user processes and hibernate the kernel session so that all the kernel threads are frozen and devices suspended when we take the snapshot of RAM contents? It saves time of initializing the whole kernel session next time we boot up. But I would like to know if there are any issues with the concept?

Are you talking about suspend to disk?
The cost there is writing that snapshot of RAM to disk. Either way it has to be read from disk on boot and that's the slow link in the chain.

I use suspend to RAM but can't see the point of suspend to disk unless you have some big desktop/application state you want to preserve.