x2 on most of what snowolf said.
I taught full time for a winter about 7 years ago at Snowshoe WV. I got back into it working part time for the last 2 years at Wisp MD.
Its selfish, but my biggest reason to get into teaching was to improve my personal riding. I was self taught and that I was a pretty decent rider. Riding with and learning from people who had been in the industry for quite some time was an eye opening and extremely positive experience. I realized I knew little to nothing about proficient snowboarding.
There are a lot of positives to teaching. Girls dig snowboard instructors, you will have an amazing opportunity to learn how to snowboard well, you will get a lot of snowboarding days in, you will make money being active doing something you love, you will get to share something you love with other people, you will probably meet a lot of interesting people.
There are negatives as well. When I started I had the idea that I would be on black diamonds everyday. That wont be the case. Starting out you will not have choice on lessons and you will most likely be stuck dealing with kids. Kids can be challenging to teach. You will probably only get beginner lessons, unless you are a very gifted rider. As Snowolf said it can be very frustrating dealing with unfit people in a lesson. Its tough when people get frustrated.
From my personal experience I think working part time is the way to go if you want to instruct. Make decent money during the week so you don't have to be stressed about making ends meet from teaching. I think each resort is differnet, but at my current mountain - the pay is nice. I probably averaged 20 bucks an hour not counting tips for the hours I worked. I find it to be a very rewarding job. I genuinely enjoy sharing my passion for snowboarding with others. Its really cool to see someone pick up a concept and be excited about progressing. Maybe its different out west, but I feel that AASI is a great organization. I only have my level 1 but plan on going for the 2 this year. Both resorts I have worked for refund your dues/exam fees at the end of the season. If it doesn't cost anything why not do AASI events and learn from the best riders in the country? You also get nice proform deals on gear.