The thing that really irked me about the civil war questline (comparable to Dawnguard in that joining one faction makes you the enemy of the other), was that it really didn't allow you to choose how to go about helping your chosen faction. You just charged into forts head on, regardless of playstyle. Enjoy stealth? Tough luck. There's next to no scouting, sabotage or spying missions - and even in those you are expected to get into full - on combat. AOE destruction/illusion? Too bad, there are loads of your own troops around.

I just hope that the Dawnguard questlines don't presume that you'll want to play every mission in a certain way.

I agree if it's another all out war like the civil war was my argonian thief/assassin won't last very long.

a valid request my friend, ale!

While I see your point, you can still be stealthy with the missions. You just have to be VERY creative. If I remember correctly, you told the troops when to attack. You could just pick off all the enemies you can, then tell the troops to charge and finish the rest.

And AoE spells are definetly out of the question.

I hope there's more opportunities to use Restoration magic like healing an important npc or they'll die, or learning a powerful expert level spell to make killing vamps easier.

While it would be nice.. That is kind of what the Thieve's Guild and Dark Brotherhood quest lines are for. For the sneaky peeps.
In a civil war it's to be expected to just kind of charge in, arrows going through the sky and swords swinging around.

Armies have scouts, and spies.

It's looks like if you're on the vampire side you got MAGIC if you choose Dawn Guard you got your WARRIOR style play and could pull off STEALTH. Both side may have everything

What about vampiric warriors? Or "holy" mages (battle priests, etc)?