First, Dualspecs won't be in for launch, but devs said they may come later.
I have tanked as a Bounty Hunter Powertech, which is the Imperial Trooper Vanguard equivalent, and it is the most fun I've had in the game. Rocket Launchers, railguns, jetpacks - awesome. (Vanguards have grenades and lightning cannons or something).
On the plus side, BH/Trooper have A LOT of AoE abilities, so it's pretty easy to hold aggro on groups - far easier than I found it to be on a Jedi Knight Guardian (caveat up to level 27 - most, maybe all, tanks get their AoE taunt at level 30).
The downside of that is they are a PITA to try to work with Crowd Control. Every CC ability in SWTOR (PvE) works like Hex - long duration, but long cooldown. If someone breaks it, you might be fucked.
While they are "ranged" tanks, the effective range is a lot shorter than you might expect - 10m. Sure you can still shoot guys from up to 30m away, but you have things like your primary aggro builders, damage debuffs, and basic AoE attacks limited to the 10m range. Furthermore, your interrupt is melee range only.