Space War: Really hard because nothing goes boom in space. You light a nuke off and you won't get a massive explosion because there is no air to create shock waves; nor any to sustain fire for long.
Manuverabilty: A real pain. Planets move. They ARE not stationary. And if you do not fly in a way so that you will line up with a planet's orbit, you will waste unnecessary fuel. Assuming you had unlimited fuel you could care less about that, but I don't think we will ever have unlimited fuel.
Warp Drive: If you went faster than the speed of light, how on earth are you going to pick up anything on your forward sensors? I know space is big and you probably won't hit anything, but you just might if your going that fast. Remember planets move, you would be flying blind until you reversed your engines to slow down! Communication waves at best travel at the speed of light. Anything hitting your ship will be like a bullet at that speed. Actually worse, stuff would probably go right through.
No Stealth Whatsoever: Any spaceship will be picked up by sensors because they all generate heat. You have seen how much we can pick up with our satellites today, a scifi species could detect even more.
Combat: Basically you may as well use giant rocks, because explosives won't work anyway.The only way they would be useful at all would be if they ONLY detonated once they penetrated an enemy spacecraft. If there's air in there, there will be a boom; So guided missiles could be effective, as well as lasers.
But Why: Because planets are always moving, there is no way to really lay claim to territory, saying this part of space is yours. Cause guess what? Wait a while and the spot where your precious solar system was will be empty. Solar systems move too! The only valid reason for war would be to assault planets. In space combat is too cumbersome and weapons are not very effective. Plus everyone will know your coming from miles away, there is no element of surprise.
Granted: If you had FTL drive, maybe you could slow down right in front of a planet and surprise it. But going that fast, chances are you'd miss it. unless you timed everything in advance.