Justinian
I like the early-game possibilities here. Obsolete's threads have shown the viability of a "slow" start revolving around super-specialists. I'd start by grabbing a religion, then going for Fishing or, more likely, Agriculture. Agri opens up AH. Hopefully the horses are close, as I will probably keep a small empire and tech to Guilds ASAP.
Dutch
I won't be playing the Dutch very much, as I find their trait setup cheesy and inappropriate given the size of their empire historically.
But, obviously, cottage spam is going to be the goal here. I'm somebody who doesn't mind coastal starts to begin with; but if you do, the Dutch UB eases the pain somewhat.
Ethiopia
I think Zara will be the first leader I play as. I don't know if Ethiopia will be a "powerhouse" civ, but they sure look
fun. I'm a big fan of beelining to Janissairies with the Ottomans. Here's another chance to play with a fun variant of the Musketman.
Creative is an under-rated trait IMO. Does it taper off sharply in the end-game? Yeah. But by that time you have reaped the benefit of a larger empire, period. Creative is like Stonehenge except twice as effective and it doesn't quit.
The Stele (Monument with a culture multiplier) is somewhat unimpressive; unless I decided to go for a cultural victory, which is a definite possibility with Zara. In fact, I see tons of possibilities here from culture to domination.
Charlemagne
While this guy seems to be a mish-mash of traits and assets that have little synergy, my jaw literally dropped when I saw the Landsknecht. This unit is an absolute monster, especially in multiplayer. +100% vs mounted
and melee?

If you've got Charlemagne near you, Muskets are going to be a big priority.
Pacal II
It looks to me like the Mayans are going to be a powerhouse civ. According to the sticky on the BtS forum, bronze isn't needed for a Holkan rush.
Unique Unit: Holkan (Spearman), immune to First Strike and can be built without copper or iron.
After a land-grab, you can settle back into cottaged cities and spam Ball Courts, a straight-up awesome UB. An empire with huge cities and super-charged commerce can pretty much take its pick of victory types.
Joao II
I've always had a love of far-flung colonies on islands across the globe. Many people hate water-based maps and coastal starts, but I really enjoy them. So it's these guys and the Ethiopians that I'm looking forward to most.
There's some uncertainty as to whether the Carrack can carry a Settler, though I suspect it can't. Thus its main use will be early over-seas conquest, either of AIs or barb cities. This means you need a thriving economy to support both an army and navy. Unfortunately Joao doesn't have traits the specifically aid in this, though the UB certainly does. Thing is, I don't know when the Customs House becomes available. Can anyone help me out?
Anyway, here's another case where I'd employ a "slow start" and grow my capital. When playing as an Imperialistic civ I tend to wait until size 4 or so to get a Settler going, so I can make full use of the hammer bonus. As for Expansive, I'm a pretty big fan of this trait. Granaries are possibly the most important building, and getting them up earlier is huge for Slavery purposes. All in all I think the Portuguese are set up to play out just like they did historically (unlike the Dutch.)