"Play what you want. Come up with a character and a personality first, and then we'll fill in the numbers and stats around it."
That's the advice I always give friends who are taking their first steps into roleplaying. Rather than worry about finding or learning the "correct" builds, I always encourage them to come up with a character concept first: can they use magic, what kind of weapons do they use, why do they risk life, limb, and personal hygiene fighting tomb-beasts instead of settling down with a nice pipe to smoke on a comfortably rustic back porch somewhere? Often the answers, from friends who don't know rpg tropes so well, would not build into especially optimized or "playable" characters. Nobody ever sat down to their first gaming table and said, "Man, my fantasy self would totally be a Bard 6/Battle Howler of Gruumsh 3/Sublime Chord 2/Spellsword 1/Eldritch knight 8!" In fact, the first time I played with Anthony (the co-author of this blog), he built a multiclass rogue/cleric that was a really cool piece of conceptual roleplaying... and quickly discovered that he was largely useless in a fight.
As a GM, you can always change your own game and write things that appropriately challenge your party. It would be entirely possible to run a whole campaign in which all the PCs were commoners and in which all of those commoners felt heroically challenged in every session, barely overcoming the obstacles put before them each week (feats such as "wrestling those escaped pigs" and "rescuing Widow Birchark's boy who climbed up the inn and is too afraid to get down again"). As they leveled up, they could probably even take out an orc or two. And finally, as 5th or 6th level commoners, these brave heroes of the town could muster their courage, grab their torches and pitchforks, and go kill that troll that's been terrorizing the village since their grandfathers' time. This would be --or at least COULD be-- a successful and entertaining game.
Now imagine those commoners in Rise of the Runelords. When the need for heroes arises and these fearless farm-raised citizens answer the call... they will be slaughtered. When James Jacobs wrote Burnt Offerings (the first module of that path) he didn't write it with a gang of commoners in mind. That much is obvious. Anybody who goes in with a gang of commoners knows what to expect: grisly horrible death, and hopefully some good laughs around the table as the ill-fated PCs stumble way in over their head and are comically disemboweled by monsters.
Obviously this is unrealistic. But now let's imagine a more plausible gang of first-time adventurers: a group of players who have just picked up the hobby and made characters based on my well-intentioned advice: "play what you want". We have a rogue who decided he wants to be an archer (he was discharged from army sniper training because he refused to kill an unarmed combatant), a middle aged sorcerer who took Youthful Appearance and Air Bubble as his spells (he comes from an island nation where people are killed when they begin to show signs of aging, and so in terror learned to mask his age until people became suspicious, then he left), a cleric whose player comes from an MMO background and thinks of himself only as a healer and so carries no weapons, and a low-strength fighter who has weapon finesse and a rapier, modeled after Arya Stark.
These people will also die horribly. They're all interesting characters (except for the cleric) but they're simply not up for the challenges of a professionally written adventure path. An amateur GM will likely murder them all almost by accident just by running the modules as written. An experienced GM will either murder them intentionally (if she's a by-the-book GM) or will spend hours bringing down the power level of the monsters they face so that the players can experience the rewarding, challenging story that their characters deserve.
What can be done about this? I have a few ideas, and many more things to say on the topic. I'm hesitant to make my first real post an enormous wall of text, so I'll break this into multiple posts with this, the problem, as the first. Consider it a cliffhanger, and if you're one of the three people that see this, feel free to let me know your experiences.