They're unprepared for the college-level coursework, financial responsibilities and freedom that they've been granted all of a sudden.
We pulled out the best replies, edited slightly for clarity:
You don't have to go to college:
"Seriously, most people don't have their stuff together at 22 (roughly when you graduate), and/or are just starting to get their stuff together around their junior year. That's tens of thousands of dollars they wasted either partying and failing classes, or not meeting people, or being in the wrong major."
"Not sure what you want to do? Not really all that in to school in general? Take a year or two, get your head on straight. If you decide to go at age 20 or 21, no one will think much of it other than you can buy alcohol earlier. So many kids go way into debt at crazy interest rates for degrees that do nothing for them, just because they can't think of anything else to do when they graduate from high school." —mmmsoap
Your professors are not your parents:
"Professors are there to teach the material and help your understanding of the material. They are not there to tell you that you're special, that you can do anything, or spoon feed you answers." —slyscafe
You won't be who you were in high school:
"The day after you graduate high school, your social standing in the class hierarchy no longer applies. It's a clean slate so don't act like you're the big man on campus." —xeskind30
Go to as many networking events as possible:
"This is just as important as going to class will be for the rest of your college career. Seriously, friendships are forged and memories are made at these stupid things. You'll still be talking about these events four years later at graduation."
"It is really tempting to settle in and nest when you first get there — unpacking your stuff, sorting out your room, etc. SKIP IT. It can wait a couple days while you run around campus doing random nonsense." —purplepeapod
Invest in your professors:
"I think getting to know professors is something most people don't realize is as beneficial as it is. This isn't just a 'know their names' because I had those. But I got close with a few teachers, and even though I wasn't an A student, they gave me good letters of recommendation. And their office was always open to me for advice, chatting, or homework help for other classes." —namer98
Get an internship:
"I interned for three out of my four years and had so many job offers after college it was insane. It gets you knowledgeable in your field, typically gives you money (don't settle on unpaid internships, they're a joke), and can give you 1-4 years of work experience which will influence your paycheck and job offers right out of college."
"I easily made double what my friends made (had 3 years of real experience), and got offers where others were still unemployed trying to desperately break into the mark. For my last year I actually was able to claim my paid internship as credits toward my degree. It's a win-win." —Scyth3
Or get a job:
"I studied engineering, but while my other classmates were excelling in class and getting way better grades than me I held a job for 2.5 years (as a videographer), joined a service fraternity, and lead a local volunteer group, and got a 3-month internship."
"I was one of the first of all my graduating classmates to get a job even though others had much higher grades. So hold any type of job, find leadership positions in anything, and have a great time in college." —ivegotagoldenticket
Learn how to write:
"A great idea and thesis doesn't mean a thing if you can't communicate the message. Seriously, learn to write well. It will help you the rest of your life for any white collar career." —b_tight
Research and understand how college loans will affect you:
"Student loans are no joke. If someone had impressed upon me the reality of starting my adult life with $100k in debt, and specifically what the monthly cost of that debt vs. what my realistic salary expectations would be, I'd have completely reconsidered how I approached college." —Gingerinthesun
Take your scholarship seriously:
"If you lose it, it's your own fault. I knew exactly what grade number I needed in each class every semester to keep my 3.25 average and not lose my scholarship." —hpstrprgmr
Get up when your alarm goes off:
"Put your alarm on the opposite side of the room. It forces you to walk when you wake up in the morning, and by the time you get there you should realize you need to stay awake." —Aptimako
Always go to class:
"Go to class. You have to take responsibility for your own future now. It's up to you whether you want to sleep all day or be successful in life. Make the right choice, even if it's the hard one."
"This is really, really hard to do when you have calculus at 9 a.m., but you just have to do it. Skipping class can be OK sometimes, but you should never make it a habit." —Punksworth
Try new things:
"Do things that you never would have done in high school. Do things that fall way outside your comfort zone. Do it because you can and because you'll never know what you love if you don't find it. And you find it by doing new things. I grew up with a mindset that you either did something perfect or you didn't do it at all. And I was scared to try new things. Go out and do new things. This is very sound advice." — StickleyMan
Make new friends and be social:
"College classes aren't like high school classes. They can be hard. You need to have friends in your classes to study with and to help you out when you don't know what you're doing. This can be vital." —Crepe_Cod