Grades K-8
The easiest of them all, right? Well, maybe...it depends. Sure, the material is not the hard part—you remember all that stuff, hopefully—actually explaining it to the kids and drilling it into their brains is the hard part. Of course doing this requires an organized and info-enriched lesson and outline...yadda yadda...but the real challenge is designing lessons that excite and keep control of the kids without them realizing it.
The willingness to get creative with grades K-8 is essential. This means that you can't be scared to "get a little dirty" in the process. If you need to make a fool out of yourself to help the kids remember something, then do it. For example, I had an inspiring Spanish I teacher in high school who would dress up in costumes and bring in props to illustrate the week's vocabulary terms to help us learn them faster. I can still see Senora Costales roller skating around the classroom. She was previously a drama teacher; can't you tell? What I mean by this is types who excel in the arts have some powerful tools to bring to the table. Humor, drawings, skits, funny voices, and music are all great tactics to invest in when teaching most subjects to grades K-8.
High School
High school teachers should apply some of the above qualities when teaching teenagers, but the most valuable element is reinforcing the students. Grades 9-12 are the middle ground between childhood and adulthood—you've got a mix of kids who are still very "young" and those who think they're 24. You really have to be versatile to teach high school. Though it depends on what grade you are handling, high school kids are usually at a variety of emotional and physical developmental stages.
As a high school teacher, you can't really take too many things seriously, in terms of what the kids do and say, but you have to know when to draw the line and discipline them. Though discipline is an issue at any education level, high school seems to be the prime for this. Many kids try to show off for the class; some have imbalances or disorders; and others can have troubles at home that are reflected in the classroom. The job of the high school teacher is to assess the student and handle situations accordingly.
College
Working as a professor can be a whole new world in comparison to K-12. Educators who are more scholarly and/or less interactive often take this route, though there are exceptions. At the college-level you are, hopefully, dealing with adults. This means less illustration and less hands-on techniques. Most university and college professors lecture and use few colorful activities and projects. Group work is also something that many professors encourage, as it gives students a chance to independently discuss what's been taught and translate it into meaningful terms for themselves.
Teaching at the college-level is great for those who have other facets of their career to attend to, meaning that, for example, many law professors are partners at firms, many theatre professors are working actors, and so on. Because the schedule can be so flexible—teaching a minimum number of units—teaching college can be a nice supplement to a steady living or the prime source of income for someone with a busy personal life. Regardless, teaching at the college-level means that you have advanced expertise in one subject, opening doors for you to publish books and studies or even serve as an expert in that field.
Alternative and Special Education
Teaching at alternative schools and in special education classes can be challenging, but those who are cut out for this type of work find it a joy. At this level, though it's important to get through the lesson plan, it's almost more important to make an impact on the students, whatever that impact may be.
Alternative schools for troubled students who have been kicked out of their original schools can present many new issues that most other teachers will never have to deal with. Students who are affiliated with gangs and drug dealers can pollute the learning environment, and it's up to their teachers to help steer them on the right path, or at least, teach them a thing or two. Discipline is an ongoing issue, as these types of students can require very close monitoring and reinforcement to get through the school day.
Special education is the same in many ways, but of course, these children do not have control over their actions in the same way. Both branches require teachers who are patient and have the ability to nurture and mentor their students. It's a more personal calling in the education field. Many people who go into these areas have family members or loved ones who fall into these categories.