I've played a bunch of sealed on the MTGO beta over the weekend and a couple drafts as well.
Boros is very difficult to handle unless you're Selesnya and/or got all the low cost removal of which there is little of. On the other hand it's not that easy to open a great boros deck in your sealed pool, more often than not I'd expect to play 3 or 4 colours in sealed w/ all the gates and green fixing.
Selesnya is fantastic against boros cause they have 2 commons and 1 rare that gain 4 life on ETB and have decent to great stats and they eventually go over the top.
Dimir is lacking great finishers. They got a couple good flyers but removal for fatties is plenty in this set.
Golgari is pretty bad on its own, undergrowth has almost no enablers. Surveil doesn't really do it since you're generally not pitching creatures into the GY to get 1 more life on ETB or 1 more power until EOT. But Golgari has great midrange cards that work very well combined w/ Selesnya.
Only Izzet deck I built had Niv Mizzet and don't know exactly what it's doing w/o such bombs. I've double splashed hypothesizzle more than once though since that card is nuts. Does come w/ extra feel bads when you whiff.
I'm probably skipping prerelease because I don't care about paper magic and Arena is going to have Sealed and I have approximately a billion gems coming my way.
did you try out the beta at all? They got rid of the anoying beta packs and it was straight GRN sealed this time.
