The bit that is missing here is that nobody has described how access may be granted (or denied) to certain classes of airspace. The basic deal is airspace Class B to E is accessible to VFR aicraft but you must have a clearance before entering and comply with clearances given inside. This means you have to have plan with and without transit because you have no right of entry. Traffic loads often mean controllers are unable to grant transit clearance. Classes F & G are uncontrolled and you don't need a clearance. In each piece of airspace different rules regarding distance from clouds and flight visibility apply so again you may be denied access for thus reason.
PM