As noted above, Grade Eleven is the last chance most students will have to make meaningful adjustments to their class standing, grades, etc. It is the year when life after high school first comes into focus, and the year when important decisions about higher education, the military, etc. begin to congeal. There is a common misconception among younger high school students that these are matters best reserved for ones senior year. In fact, decisions about ones future are usually made BEFORE the beginning of the senior year. That year is spent with applications for college admissions, scholarships, loans, meetings with military recruiters, etc. For that reason, the eleventh grade is the last best chance one has to adjust his course in life before one leaves high school.

I would agree with post 3.  Grade 11 is the most important year because you will be using the grades, test scores (SAT and/or ACT), and other achievements to secure a place at college.  You will start applying for higher learning institutes somewhere in the beginning of your senior year (or at the end of grade 11 if you are going for early admission).  You will be using your grades and achievements from Grade 11 because you will be currently in Grade 12.  This doesn't mean that Grade 12 doesn't count.  The colleges will look at those scores before you enter their school in the fall.  They can still turn you down or change your registration status if those grades are not up to par.  However, the first thing they will see about you is all the information from the completed junior year.

There is clearly a step up between tenth and eleventh grade.  Coursework becomes more analytical and challenging in order to bring the student to the level of thinking at which he/she will need to be for college. Also, as mentioned above, the eleventh grade is the litmus test for a student as it is the final one that prospective scholarship organizations will be evaluating.

Let's face it, the next year, your senior year, is going to be a battle with senior-itis. If you don't do well in 11th grade, you're putting yourself in a position of having to make up ground when it is most unlikely that you'll want to work that hard. As the other posts mentioned, 11th grade is the last full year that colleges will see on your transcript, unless you skip a year before going to college.

I would also suggest that this year (the 11th) is important because it is when one typically takes the tests required for college entry (the ACT or SAT). Outside of that, many students begin to look at colleges more seriously than they have in the past, if they have in the past.

I think it's because it's the last full year of grades that are going to be on your transcript when you apply to colleges.  If you've done well up to now, you still have to do well this year so you don't look like you're dropping off.  If you've done poorly, you can improve in 11th grade and show colleges you're getting better.