It's easy to rush a base, we all know that. Get a force coordinated enough, and it's neigh impossible to stop. Spam turrets and hydras, and you burn precious res preparing for a rush that might not come. Marines spread out across the map trying to build nodes? Rush the powernode then the chair, nearly insta-game. Got shotty and aliens about pestering nodes you placed to distract em? Beacon back and shotgun rush the hive. Easy to end games in under a couple minutes. Basically, as I see it, the major difficulty early game with defending is either expanding too quickly trying to keep up with your enemy, or not expanding much at all trying to move your slow ass to a place to defend, only to arrive finding the remains.
I've got a simple solution for aliens anti-rush: creep is very poisonous without a mid tech tree upgrade (moderately inexpensive to research, arms lab).
As for the marines, that's a little harder as their style doesn't include covering the map in creep. I'd say bases with comm chairs have all buildings within that sector electrify attacking aliens until "grounding" is researched from a shell (Takes a while to research, moderately expensive). Simply touching a building would electrify (prevents IP camping). Also, extractors can have res spent for this upgrade as well. This isn't your NS1 style electrify either, this would be a much more dangerous version.
Since most of the magic with marines happens in one base anyways, whereas the aliens spread over the map, I think both answers would fit their styles.