Ender's game also had that tactic of huddling around the leader than unleashing the death ray; due to the 3D nature of space and drones making a ball of drones around a important spaceship the ship with the death ray thing managed to haul ass through all the bugs.

From what I can tell, it's range and stealth if you wanna be all "hard sci-fi" about it since space is vast, making detection hard as all hell. I believe a interstellar society would be very, very hard to exterminate just because of this; they could hide out in the outer ice belts and keep system hopping at sub-luminal speeds or hide out at a planemo or have a hidden underground moon base. Death stars also would have a hard time snuffing out space stations hidden in asteroid belts. There's lots of space and time to do stuff like this, and technology by that point likely will make drilling through rock and making a moon base as easy as making a mine is today. Space being the 3-dimensional void it is really does wonders when it comes to evasion. In a sense I believe hit and run attacks are a good idea in space combat; not to mention it leads to psychological warfare by instilling paranoia in the enemy which could result in a misfire on their part. How would they know when you will strike, if they can't detect you?

Yet at the same time it's kinda scary how easy killing of a entire race can be once you can glass worlds or terraform a world's atmosphere to make it uninhabitable. It seems in the interstellar world it's go to space or go to the intergalactic crypt.

On the offensive I am most worried about planetary settlements; someone could just toss a few rocks at a world they don't like and kill hundreds of thousands, if not millions of people. Ship-ship combat would be dull, and paranoia inducing at the same time since you don't know if the enemy has a lock on you or not. For that reason you'd constantly need be on the move. Rail guns I used to think would be cool, but I imagine that you'd need a really high power kinetic to rapidly cover the needed distances while inflicting maximum damage. It should also take into account that the projectiles don't necessarily need to be dumb projectiles. 

Of course being realistic about this is hard; for all I know particle beams and energy wave projectors [a weapon that unleashes a wave of deadly plasma basically, sorta like high tech shotgun for spaceships] may actually be very viable weapons for a interstellar civilization. Trying to imagine what weapons in ship combat will be like can be hard since I don't think it is even known what sort of weapon is the most practical. And i've seen people who think everything from "plasma is a waste of energy" or "particle beams take too much effort" or "kinetic weapons will be useless in space". No one really knows; so I believe that the answer will likely be "all of the above" more than anything else since technology is a strange, unpredictable thing.

In a sci-fi setting where asteroids are depicted way closer together than they really are [at least normally], I could see a spaceship shooting a asteroid into another asteroid to set off a chain-reaction of moving asteroids to catch a enemy off-guard, or hell maybe even have a probe drill into a asteroid and pilot that at the enemy. Hell, why not go all the way and make a warship out of a asteroid? That would be cool, as asteroids give natural armor and some quick resources.