Where do I begin? First off, so much depends on the school district you work for and also what area you plan on teaching.
I'll start off first with the negatives:
1) You will have a wide range of students, from very high learners to very low, from very appreciative students to very rude, from very disciplined to very, very undisciplined, those from good families and those from very poor, struggling families. In sum, you can get the whole spectrum. And you find that you spend a very good portion of your time dealing with those students who are disrupting class for whatever the reason and, as a result, less gets done in the 45-50 minutes of class you have each day. And that adds up over teh course of a 180 day year.
2) You may not have the autonomy you desire. I'm fortunate that I'm the only teacher of my discipline in the district (I teach Latin and Greek) so I can set up the curriculum to what works and to what I want and what the students often want. Most teachers don't have that and must follow a pre-set curriculum that perhaps has been unchanged for the past 10 years!
3) Politics. It's always there, sorry.
4) Paperwork. So much time goes into bureaucratic, redundant paperwork. This includes referral forms (tardy, absences, behavioiur), SPED sheets (we call them rainbow sheets), SAP (student assistant team) referrals, etc. This can really add up.
5) Contact with parents because of a bad incident. Most of the time parents are very appreciative and thankful that you let them know about such things, but then there are those for whom their kid can do no wrong and then assume you're out to get them.
6) You realize just how dumbed down society has become by observing students' behaviour in the halls. You want to do something about it, but, in the end, it becomes a losing battle.
7) No matter how good a job you do, there are always mediocre colleagues that always seem to get the accolades that you deserve. It gets frustrating.
Depending on the school district, you may or may not have access to basic technological needs.
9) Depending on the school district, you may have to comply with a grading protocol that doesn't assign 0s or gives students an exorbitant number of chances to make up work, missed tests and quizzes, etc.
1) As corny as it sounds, at the end of the day, you know you did something worthwhile.
2) I've developed long-lasting relationships with any number of my students and their families.
3) You get to do far more than teach your subject but mentor these students about life and general.
4) The hours are great.
5) I'm fortunate to have a good pension plan and benefits plan.
6) It gives you a chance to be creative and try new things.
If this is something you wish to do, let me give you one lasting piece of advice: Never take your work home with you. That includes papers and also any emotional baggage. Home is not school, ever!
Hope this helps and good luck to you.