There are MANY nations and sub-themes, and the game is new. It's going to take a while to get a solid grip on what's good and bad.
That said... this is my gut reaction to the major nations/themes;
Abysia - Slow starter, but good endgame. Blood magic ramps up nice. Don't underestimate the Warlocks.
Abysia, Blood of Humans - Weak. I don't like it. The full game doesn't seem to be enforcing scales like the demo. A change between Versions? Even so, there's nothing that gets my attention.
Atlantis - Like the defense of starting underwater. Like the Dagon. Good magics. A strong nation from start to finish.
Pythium - Good troops and good magic. Even without "Gateway V.1" Pythium still has lots of what it takes to win, and no real weakness.
Pythium, Serpent Cult - The Hydra's can be abused with the new bless effects. However the magics are weak. I don't like having all eggs in one basket. It's got power, but weakness as well. Not as good as regular Pythium. IMHO.
Man - Good fast expansion. Nice spells. Crones still rock. Stronger than Dom1. Now a real contender.
Man, Last of the Tuatha - Not bad, but I like the crones better than the Tuatha mage selection.
Ulm - Was second rate in Dom1 and may be thrid rate in Dom2. Smiths now very weak. Total lack of ability to handle super combants will make life hard on multiplayer Ulm.
C'tis - Death magic not hammered as bad as other paths. C'tis comes out way ahead compared to Dom1. More complex than some other nations, but there is potency in the Dom2 lizards.
C'tis, Desert Tombs - The theme is expensive. Takes a bit too long to ramp up the free summons. I think I could do better undead work using the Sauromancers from regular C'tis. It's a cute theme, and it might have a place in low resource scenarios... but in general I'd say it's not a top competitor.
C'tis, Miasma - I don't like the mashmaster. The disease effect isn't strong enough to matter much. I prefer the regular theme.
Arcoscephale - Good troops. Elephants rock. Good magic. Always a contender.
Caelum - Mammoths rock. Even without air elementals, the high seraphs are great. The mobility of the flyers will be stronger than in Dom1 due to the slower strategic moves of many enemy troops. Some weakness versus super combatants, however the sheer speed of Caelum may make it a terror in multiplayer.
Caelum, Return of the Raptors - I don't like losing air magic in favor of a tidbit of death magic. I don't see any real advantage in this theme over the regular caelum. The Raven Guard are nice, but not enough to convice me to drop the air magic.
Ermor, Broken Empire - This is a meaty theme! It's got good troops. It's got evil priests summoning undead. Very playable.
Ermor, Ashen Empire - I.E. DomI Ermor. This Ermor's dominion always attracts heat in multiplayer. In Dom1 Ermor could field a serious super combatant when others couldn't imagine doing the same. Now it's not so unique. Also the new bless/sacred system will encourage players to have lots of priest power floating around. I don't like Ashen Empire for multiplayer. It's great for single play, as the computer AI can't handle the deadly dominion.
Ermor, Soul Gate - So far this looks similar to Ashen Empire, just using incorporeal undead instead of skeltons/zombies. If someone can find a reason for this theme... let me know.
Marignon - All things considered I think Marignon is stronger in Dom2. Not hugely, but enough. It's good, but not great.
Marignon, Diabolical Faith - I don't like that it requires a minium Turmoil +1 and Heat +1. The access to blood magic is sweet, but the economic hardship is rough. I'll pass on this theme.
Marignon, Fires of Faith - Heat +1, yuk. Just to get the effect of a level-1 priest preaching in your provinces? Is there something that I'm missing? Doesn't work for me.
Pangaea - Weaker troops but much better magic. This looks to be pretty dedicated to the maenad theme. Could be OK. Doesn't leap out at me.
Pangaea, Carrion Woods - Pumping carrion creatures is about as effective as the meanad theme. Better quality, fewer numbers. It's alright.
Pangaea, New Era - Stronger troops, but less magic than other Pangaean themes. Very nice ground pounders. Some weakness versus super combatants. Not quite a top contender, but solid.
Vanheim - Loss of lesser air elementals hurt Vanhiem more than others air Users. Valks fly. It's got air and earth magic. Not sure if the new sacred/bless system helped the Van. All said and done, Vanheim is good, but not great. Unless I'm missing something.
Vanheim, Helheim - A bit of death magic? Nah. Does nothing for me.
Vanhiem, Midgard - Level 2 astral mage. Air 2 mage with 2 random picks (for 160 gold). Yummy! I like this theme. No valks though. I think this is the strongest Vanheim theme.
Jotunheim, Iron Woods - Effectively this is the "goblin" theme. Bleh. Nothing to see here.
Jotunheim, Neifelheim - Big freaky giants! A potential super combatant that I can buy for gold. Nice! This theme has some potential.
Jotunheim, Utgard - Siethkona. Real death magic. Some astral also. Probably the most well rounded Jotunhiem theme. This is the theme I would play right now. Strong.
R'lyeh - Much more "Cthulhu" flavor than Dom1. Protected water start. Looks like the mind bLasts are dangerous this time around. I need more practice with this new theme. It's got potential.
Mictlan - Free slaves. They're garbage, but hey, it's *free* garbage. Blood magic, nice. Good, but expensive access to magic paths. The eagle warriors interest me. I need to do some work with Mictlan before real judgement... but right now I like what I see. It's a complex nation. If you are new to the game, move along.
T'ien Ch'i - I like the diverse military. Magic is a bit weak, but has interesting summons. It's at least good. Possibly more...
T'ien Ch'i, Spring and Autumn - Requires turmoil +1. Has weaker military but stronger magics than regular theme. Not as interesting, to me. The forced turmoil is too harsh.
T'ien Ch'i, Barbarian Kings - Better military... but turmoil +2. Weak economics self-defeats the improved military. No interest from me.
Machaka - Diverse troops. Outstanding assassins. The magic is a bit weak, but adequate. Good, but not great. Lot's of fun though. Can't beat giant spiders for sheer smile factor.
That's my preliminary thoughts. More details as I discover them.