So.. through being extremely lucky (knowing the right person, being in right place and right time, etc..) I actually got to draft Theros last night with real cards. Went U/R aggro but learned quite a few things seeing the cards played out. Warning: incoming lengthy post.
First of all, bestow and heroic really are NUTS in limited. The removal sucks in this set, and it's expensive even when it's good. Most good decks are going to revolve around 'build-a-fatty' style play. Even in draft, which is faster then sealed, I think it would have been correct to play 22 spells/18 lands if you aren't playing mana ramp effects. You want to hit your first 4-5 land drops pretty quickly. Nothing significant happens in the first few turns barring some crazy draws. Even then, if your opponent is playing green and/or white, they have access to enough lifelink effects on big creatures or lifegain effects that you can't put them away quick enough.
The quickest game was one where I curved out on making a flyer dude huge with ordeals + bestow effects that outclassed their removal.
On to specific cards that under/over performed.
Overperformed:
God's Willing- holy crap is this card amazing in this format. Since most of the true removal spells without drawbacks cost 4+ mana, this is often a counterspell with scry 1, for a single mana. It's a huge tempo swing, and often you are protecting a heroic guy so he gets an extra trigger as well. The two decks that got to the finals undefeated were running 2 and 3 of this spell.
Hopeful Eidolon- This guy is great in a deck with even a marginal amount of heroic creatures. +1/+1 and lifelink for 3W might not seem great, but triggering heroic and getting lifelink on a fatty just completely stabilizes against any super aggro decks. He's also a good target for other enchantments so you can make him into a relevant threat. So, even if they remove the guy he is bestowing, you still have someone left on the board to build up into a fatty with your other effects.
Every heroic guy: We knew they were gonna be good, but the ones that get bigger are almost unstoppable. The centaur that gets +3/+3 and the warrior that gets +2/+2 are broken in the right deck, and all the white ones are playable even without heroic triggering. Most of the ones with effect triggers are still very good, such as wavecrash triton (blue guy that taps down a creature for a turn when heroic triggers) as he can help swing combat math when both sides have huge fatties staring each other down afraid to attack for fear of dying on the crack back. One of the weakest ones is probably the rare red guy, Labyrinth Champion. I drafted him, and 2 damage is rarely relevant to anything by the time he hits the board. Only on a super aggressive start, with very cheap enchantments/combat tricks and then just aiming the damage at their dome would he be any good. He's like a 7th pick in the 2nd/3rd packs when you know you are building that type of deck.
Ordeals of XX- Some neat interactions with these, if you are like me and didn't read them correctly the first time. If you have two in hand, cast one on a creature, then next turn cast the other and attack.. both will immediately trigger so they didn't get to 'plan around' the 2nd one. So, they become effectively sorceries in regards to their sac effect and +1/+1 counter to a creature, makes the black and green one look a little better. Also, if a creature has two counters already and then you cast a 2nd ordeal on him, they'll both trigger and he'll end up with four +1/+1 counters left afterwards. If you cast it on a heroic guy who already has three +1/+1 counters, it's the same thing. Note: The counters stay on the creature after he's gone, that's probably obvious to many of you, but a few of the players made that mistake when seeing them for the first time until they saw them in play. In any case, these are all very good, red/blue/white being the best, but green/black are both very playable as well. I'd take the R/W/U ones anywhere from 2-5 in a draft, G/B 3-7 depending on your deck style. In a sealed, I'd play any of them after fitting all your bestow guys in.
Sedge Scorpion(or any guy with deathtouch) - Things get huge quickly, this guy is one of the cheapest deterrents around. Play all that you open if you are in green. Trust me.
Benthic Giant- This guy is crazy. Take him early, or play any you open and load up on enchantments/bestow effects for him. I had two in my deck, and they single handedly won 4 games for me with my opponent showing me a hand full of removal afterwards. Only time I got one out and didn't win, was against the guy who won the tournament with a G/W heroic deck, where he just made guys bigger, faster. Got mine to a 9/10 flying, draw a card when he hits monstrosity. He then loaded up some guys and swung with a 11/12 lifelinking, reach and a 10/10 trampler.. when he was at 30 life after going to 4 early on..
Dissolve- Most deck's key moments are at 4+ mana, and often they are casting 6 mana+ spells. Cancel would be waaay better in this format then most previous ones, and this has the bonus of scry 1. Counterspells aren't normally amazing in limited formats, but this is an exception. Annul is also very playable if you want to be control-ish.
Underperformed
Spark jolt- I knew it wasn't going to be amazing, but it's practically unplayable. Even most of the 1 drops that get played in this set have 2 toughness, so you really can't use it to remove something early game. As a combat trick, it's horrible because rarely is one creature only outclassed by 1 toughness or power. Often, you might make a guy into some 6/6 monstrosity to outclass their 4/4 guy, then they make their 4/4 guy (or some other creature) into a 8/8 or bigger. The format is very much a 'haymaker' style format, where you want to be aggressively playing out bigger threats then your opponent.
Scry 1 effects- hard to explain this one.. I wasn't exactly overvaluing scry, but scry 1 really does mess with your head. It's generally a bonus on a spell you would play anyways, but considering how mana heavy this format is, if you see a land when you could use either 1 more mana to cast a big bestow guy or you need a removal/bounce spell for the current board state makes the decision pretty hard. It's still gambling on what the next card is. You get frustrated that you can't just draw that card. That said, omenspeaker is amazing and scry 2 (or more) effects are very good. Basically, if you see a cheap card with scry 1 on it, pretend it doesn't have scry 1 and ask yourself if you'd play it anyway. If not, don't include it in your deck. (i.e. spark jolt and lost in the labyrinth, which I messed up on as my 22nd and 23rd cards, partially to support two spellheart chimeras and trigger heroic effects in a pinch, but I should have just had two more creatures/enchantments)
Rage of Purphoros- Maybe it was just shitty luck on my part, but whenever I could cast this to remove a guy, they had God's Willing (or some other combat trick) to save their guy. So, I'm spending 5 mana to their 1 or 2, after they just used the rest of it to lay out a threat. On the same note, it seems pretty hard to build a reactive control deck in this format. You rarely are dealing with a threat with a more efficient answer, which is the basis of control decks working.
Tl:dr This IS a slower format in that you want to get to 4+ mana as quickly as possible, but the games can end VERY quickly once it does get to that point. If you want to go super aggro, it's still viable, but you need some mid range tempo stuff and to have ways to deal with big fatties with lifelink. For example, grabbing bestow guys (the 2/2 first strike for 2R is good for this) that are aggressive if cast as a creature and cheap heroic guys that get bigger is the way to go to be 'aggro'. I got people down to 5 or less life multiple times before they suited up a big dude with lifelink and bounced right back up to 14+ life. I only had one voyage's end (bounce) and would never draw it at the right time. One mistake or hole in your defense like that and the game is often over.
Oh, and my new color ranking is G/U/W/R/B, with green and blue being very close. Black is actually very good, but it's best commons require BB to cast, so you have to commit heavily to black. the disparity between the colors isn't that huge though. Picking black for the prerelease is not a bad idea, even though it's sort of the 'weakest' color imo, you automatically get more black cards in the promo pack so it has high risk/reward. Plus, red's promo lets you keep people off big mana so that's a viable strategy as well. I'm personally going to pick green, as it goes well with every other color in different themes.