Jump nodes are obviously the most important element in a FS universe. If you're defending, its a choke point you know the enemy have to brave to try and deploy in the system, or if you're attacking, you need to have it secured, or you will be cut off.
In a real solar system, planets move, and assumably so do installations, due to a suns gravity. This makes front lines redundant, as the 'front' rotates around a star. Also, subspace travel, as discussed, further removes the theory of a 'front' from a FS system battle.
So, it's safe to say that a fleets first priority on attack/defence is to secure the nodes leading into/out of the system. After that, secure planets and infrastructure that can support/hinder you; shipyards, supply depots, communication arrays, etc. Throughout this, you would be want captial ship superiorty - if you've secured/defending a supply depot with a cruiser or two, and a destroyer drops in you're up the creek without a paddle. This could be gained either through straight combat - destroying the enemies ships, or tying up the enemies resources with harassing tactics, forcing them to spread out and defend as much of their 'territory' as possible.
As for subspace travel within a system, you'd have to scout most positional movements with recon fighters to make sure you weren't jumping into an ambush. Lead scouts seems an obvious choice.