It blows my mind that RPG makers act like a Cargo Cult about this, especially outside of tabletop RPGs. Class is a tool that does specific things. Level is another tool that does specific things. A competent designer recognizes this fact and considers what things he wants done out of his design, and then he considers what tools can be used to achieve those ends.
If you're using Levels as a gatekeeping function for content access, why not consider Achievements? If you're using Classes, what are you using them for? Occupations? Iconic Archetypes? Artificial division of team labor? Why do you need that function out of Classes? You see what I'm getting at: you have GOT to consider what you want the mechanics to do when players use them. Call of Cthulhu, Amber, and Traveller have neither classes nor levels, and those RPGs are proven classics. 'Nuff said.