The main problem with the rebels in Egypt, is the utter lack of organized leadership. Most groups, including the MB, have been repressed into non-existance. There are no women's groups, student groups, intellectual groups etc... All attempts to meet would result in arrests prior to this. The people protesting in the streets are effectively leaderless. There might be some newly minted groups, but nothing with the type of power and experience to take over should Mubarak leave office.
The military in Egypt knows this, which is why they're standing on the sidelines and doing nothing more than keeping opposing parties apart.
The USA knows this, and also knows that it's been sending 1.5B$/year to Egypt (1.3B$ going to the military), and so is unwilling to say more than just a 'keep the peace' or 'peaceful transition' stance.
Israel knows this, and also knows that Mubarak is the main thing keeping Egyptian tanks from their borders.
The people on the streets don't care and aren't really thinking straight... they think that the moment Mubarak leaves that all will be honky-dorey and democracy will reign. It ain't gonna happen.
If/when Mubarak steps down... there will be chaos. They don't have a proper opposition party able to take over the reigns. All they'll have is a sudden vacuum with groups like the MB, and the like trying to grab control....utter chaos.