The Romans (Byzantium) and the Persian said this and look how they were defeated under just one leadership named Omar Ibn Khattab. The Arabs began to expand after the two of the strongest empire collapsed after thousand years of holding from South Europe, North Africa to South Asia.
You do have a point but everyone said their plan is a joke and it won't go far. This happened in 2006 when Islamic State of Iraq (ISI) were just a small group operating in Anbar against the US force, Zarqawi was killed and the organization was beaten by the US backed Suuni tribes known as "Sahwat" who expelled them to the desert. When the US left the country, the Shia dominated Iraqi government backstabbed the Suuni civilians and turned Baghdad into a Shia majority through ethnic cleansing (search Iraq Death Squad). This led to Suuni wanting ISI to come back due to their strong anti-Shia root, since after the first two years of the Syrian civil war; they came back and announced the Islamic State of Iraq and Sham. Again, everyone called them a joke who won't make it far, look where they ended up; the entire cities of Western and West-Northern Iraq collapsed since January, that's 6 months (including East-Northern Syria). When the Iraqi army had been given billions of sophiscated arms and trained for years yet ran like a pussy? This will undermine the US on how chitty they are when it comes to training.
These guys must be laughing at us how we mocked them from the start...
They have gained popularity due to their social networking so you can't tell who is supporting this or that; the OP posted the Lebanese celebration on the unexpected announcement; it happened in Jordan as well so it's not about the strength or capability they have but reputation and popularity will lead to uprising against the current governments.
Not to mention, if Al Qaeda and other jihadi factions who operate actively in Yemen, Somalia, Algeria, Sinai, Afghanistan, Chechyna, Mali and other countries pledge allegiance with Baghdadi then expect a major change in the Middle East and Africa because they now hold borders than can go through Saudi Arabia, Jordan and Iran.