The Iranian left also had no blueprint for moving forward. They were able to destabilize and oust the Shah, but they didn't know how to fill the power vacuum. The mosques did. They were not a marginalized fringe political wing, they were a cultural and religious force that quickly leveraged their position to move into the political sphere.
Thirty years later, the situation in Egypt is different. The secular centrists and left-wing have a clear model of how to fill the vacuum. Even if they didn't before, Tunisia has provided one for them. There is little opportunity for the Muslim Brotherhood to step in as strongmen willing to make order out of chaos.
1979 Iran and 2011 Egypt are simply materially incomparable situations. There is a possibility that the MB will cause headaches for an emergent democracy, but their ability to seize control and establish a theocratic dictatorship is minuscule, at best.