The three legitimate jobs I've held have all been a mix of good and not-so-good mentors. In one, my boss (mentor 1) was superb, but my trainer (mentor 2) was not. From these experiences, I've found that having a good mentor makes a dramatic impact on the position's productivity, enjoyableness, and understanding.
Below I lay out—from my experience as both mentor and mentee—the qualities of a good mentor and their importance.
Mentor should make sure mentee is comfortable telling them anything (job-related!). Mentor should never laugh at, scoff at, belittle, get angry at, etc. mentee for asking/telling them something.
Making time for mentee on both a regular basis and whenever they need mentor (see above). This can include scheduling daily/weekly/monthly/quarterly/biannual/annual one-on-ones.
Mentor should also make no indication they are annoyed by the time mentee is taking up. Replying along the lines of "I'm a bit busy right now, but can help out at XX:XXpm" is appropriate; groaning and asking "what is it now?!" is inappropriate.
Mentor should take an active interest in mentee's progress in the role. This does not mean they should micro-manage and babysit, but rather periodically check in to verify both parties' satisfaction of mentee's progress. Mentor should a) want mentee to succeed, and b) make that abundantly clear.
Note that comfort, time, and interest all relate to one another: mentee is comfortable in asking for mentor's time and giving honest feedback about progress. Additionally, superb mentoring is a help-you-help-me situation. Mentee is likely to contribute more to the group, and in turn, mentor's work.