I can understand the joy of playing a character straight out of the book, 1-20, as a member of that class. The thing is, the mentality behind this is often "Oh sweet, I can be one of these classes!" My mentality, on the other hand, is "Oh sweet, I can be any of these classes! I can be ALL of these classes!"
Those don't seem different, but they are. A new player will often have a character in mind but not a character on paper. They'll want to be the guy who runs around singalinging all over the place, so they'll play a bard. But they won't particularly want to cast magic, and sneaking's not their style. They like fighting with axes. So rather than do something like Bard3/Marshall2/Crusader15, and have a character who has Inspire Courage, and other helpful auras/crap and is able to hit things, they'll be Bard20 and not touch parts of their class features. Which is sad, because the second one is really much closer to what they want to be. On the other hand, sometimes Wizard 20 or Monk 20 is just what you want.
As an experienced player, you will find that you will not want to limit yourself to just one class. If you have a truly outlandish character concept that you must bring to fruition, for the art and science of it, it won't matter how ugly Factotum3/Monk2/Marshall1/Paladin of Freedom2/Hexblade3 looks written on your character sheet. To you, it'll say Eldrin Worstford, the burned out elven cassanova who learned a long time ago that trust is for suckers, and who does not let on to anyone at all that he is not a weak old man, as he claims, but is in fact rather capable, out of fear that if it is discovered that he is strong, he will be killed.
I made that character. True story. Mechanically, he's basically a lockdown tripper. Could I have done the same thing mechanically as a Fighter 10? Yeah. Probably better, too. He'd have more HP, AC, Etc... but I wanted to make a guy who convinces everyone that he is a weak old man, but is actually very strong, with phenomenal + 20something saves and loads of HPs and +16 bonus to tripping people... as a basic Fighter or Paladin I could've gotten close, but close isn't good enough for me.
As an experienced player, you don't see classes, or levels, or feats... you see characters. A character has all of these, but he defines what they mean, not the other way around.