The following are the misconceptions that are usual from prospective and first year students.
Wanting to teach is a good reason to get at PhD. A PhD is a research degree. Most programs do little to prepare you to be a teacher. Many people go on to teach as part of their job, but those who become the best teachers do so because of additional training acquired outside of their PhD training.
Grades are important. Doctoral programs are not undergrad. Grades are important to the extent that you learn the material and advance in the program. Almost no one will ever ask a PhD graduate about their GPA in their doctoral program. They will ask about publications and grant applications. They will ask about skills they have. As such, it would be much more advisable seeking manuscript writing, analysis, and/or grant writing experiences than focus 100% on coursework.
Your work is good enough. Let’s try to be rude or cute here, but it is amazing how much resistance we receive from students when we critique their work. If someone who has achieved success in their field provides constructive criticism of your work, you should take it. This goes for anyone (faculty included), but especially trainees. Many times students are upset that their work has been criticized and/or their brilliance isn’t being acknowledged. Odds are, your brilliance is a delusion of grandeur. Try to understand why the criticism is correct and improve you work. If someone is critiquing your work, it’s usually because they see promise in you or your ideas. Let them help you reach your potential.
Ph.D. is for young people. One of the biggest misconceptions that we use to encounter is that PhD study is only for young people. Even at 40 you can begin a PhD as long there are good reasons behind.
Ph.D. guarantees job. Another misconception from many PhD students that their degree will guarantee them a job. You have to make your fortune and your future. People think that you can get a non-exploitive teaching job, in your field, at an institution in an attractive location. You might get one of the three, but only the absolute cream of the crop can land tenure-track jobs, with good research support, teaching their specialty at a good school. Many, many PhDs are adjuncts or something like it, or else tenure-track in a middle of nowhere or the equivalent, teaching heavy multi-prep loads to students who don’t care, while desperately trying to publish their way to something better.
The job prospects are uniformly good or uniformly bad. In some schools and/or some departments and/or some subfields, students can find academic jobs with a relatively high degree of certainty, especially if they are willing to lower their standards. In other departments and subfields, academic jobs are hard to come by, even for the very best students. Ditto for non-academic jobs. Some fields feed naturally into industry, and in others, students have to work harder.