See above, Leyline isn't even good enough anymore with Reverent Silence around....Leyline in play, Cast Rev Si, Opponent goes FOW, you go FOW/Pact, Flash, GG.
That's been my experience too. I've been pushing 4 Reverent Silence and 4 Massacre on the board since they deal with the most played problematic cards (Leyline and Mindcensor).
But what do you take out to bring all that in? Plus, Leyline and Mindcensor aren't the only problems. If you're facing Mindcensors, then you're probably facing Bomberman, which means you'll need to deal with Engineered Explosives for 1, and possibly even Chalice at 2. Reverent Silence and Massacre don't do anything against those, so now you'll need to bring in something to deal with artifacts, too. How do you find room in Flash to bring in all this anti-hate?
I played Flash at a tournament last weekend (report
here), and this is what I encountered. In several games, I was prevented from comboing out by Spheres of Resistence, Glowriders (!), Grand Arbiter Augustin (!), Engineered Explosives, Aven Mindcensors, and of course Leylines. Most of the time, I didn't even bring in Reverent Silence, opting instead for more bounce (Chain of Vapor, Echoing Truth), since they would at least let me deal with whichever particular hate my opponent happened to land first. But that increased the demands on my mana.
In a 59 player event, with allegedly 7 Flash decks in attendance, none of them made the top-8. However, that may at least in part be due to a metagame that was expecting lots of Flash, and was ready for it.
