Right in the first world, you'll have to contend with building
upward momentum in an open space where the
slightest tilt to the left or right can bring all your efforts down; using balloons to hold your bridge up over a gap, but making sure they don't float too high so they don't pop on the spikes above; and collecting stubborn immobile Goo creatures who can't be picked up via touch or motion controls but are vital to the required end - level total.