I've always assumed I was just going to level as combat since that's been my beloved spec since vanilla. But, if anything leveling 85 to 90 as a combat rogue has shown me just how horrible they scale. Currently combat is arguably the highest dps raid spec which should translate to an easy face roll leveling spec with awesome gear right? I don't think this will be the case especially considering combats heavy reliance on things like the AoC and 4 pc. tier set combination as well as ridiculous levels of haste (seriously go play your heroic geared combat rogue and then try to play a freshly dinged 90 it's horrible).
Currently on the timeless isle while killing big elites I find myself having to rotate cooldowns between them to have any sort of decent killing time as combat. And this is ok since I have an AoC and my CDs are up pretty much every elite and I can just rotate through them. Comparatively I can just burst something down within my opening FW period with sub or assassination just has steady high damage and steam rolls with stuff like MfD and deadly momentum.
Currently it really doesn't seem like there's going to be many changes to the rogue class aside from the removal of shadow blades and the addition of CP on the rogue (hurray!) so I'm assuming that game play is going to stay relatively the same.
Combat has many many band aids on it currently that get ripped to shreds once you start leveling. I have to assume the AoC will get some sort of rating on it that by 100 will be dog shit other wise it will continue to be the BiS trinket for the rest of time so as you level as combat you're eventually going to lose your 4 pc., the AoC will diminish to bad and your haste % will drop back down leaving you a sad sad combat rogue.
Sub on the other hand will have high burst at the start and you'll pretty much melt mobs as you start to level. With the change to shadow dance you'll actually be able to use it solo! Woo! And it has an innately low CD so you will frequently be able to drop any named quest mob you run into. But, as enemies get higher and higher HP pools and your stats diminish come close to 90 you'll end up hemo spamming your life away while mobs take longer and longer to die.
Assassination on the other hand seems like it's the best scaling spec at lower levels. You have compident aoe with all your poison damage and if MoP has anything to say assassination seems like it's the higher damage dealing spec at low item levels. But, it has a lack of burst out side of Vendetta. The addition of MfD and cp on the rogue will make leveling as assassination so much easier and you won't have the giant ramp up time that assassination currently has but should be able to go from mob to mob much easier.
Personally I'm think that I'll dual spec sub/assassination and start off as sub while I can still burst mobs dead in a few seconds and then once it gets a little longer swap over to assassination.
What about you all?