First go read this: here. Back? Good.
Now some notes: Depending on the system it will still be possible to make "deep space" bookmarks. Just not EXTREME deep space bookmarks. Basically a deep space bookmark is defined as any location you can warp to that's over 14 au from any other celestial object. Now people have in the past taken this to extremes using various techniques in order to get very very far from a star in order to place a deep safe that required minutes in warp to get to and from (and that's if you had the cap).
These can make invasions much safer since for example if you know there's "super deep safe", you can have a cyno cov-ops get to it then light the cyno well out of the solar system. This will allow your cap fleet time to all jump in, and load grid before any defenders can warp to the grid they are now on. Which in theory allows the attacker to become the defender due to the defenders now having to load the grid while the attackers are already loaded. We all know how the current asymmetrical lag advantages the side that already has the grid loaded.
So that covers the generic use of the super deep safe - not to mention it's just about un-probable since you have no way of knowing (short of spies) about where it's located if there's no cyno up even when there's an un-cloaked ship at it (insert "space is big" reference here).
Now a normal deep safe is simply one that is outside of d-scan range of all celestial bodies. We'll still be able to make those depending on system geometry. But they will be probe-able. 64 au (for combat probes) from both ends of a line between two celestial bodies is 128 au. Very few systems have bodies more than that far apart. And the spots that are - good probers will make sure to place a deep space probe in likely spots.
So overall this removes a technique that is very close to being an exploit (constantly logging on and off for the emergency warp trick is an exploit any way you slice it). So closing down these exploits is good. The only people who are going to whine about this one are people using these exploits anyways. Talk about crocodile tears...
If you want safe - fly a cloaked ship (and I'm sure that CCP is working on some underhanded module that will be like active sonar and be useful for detecting cloaked ships one day).