It's fun to work in Thailand and once you jump through all the hoops and do the run arounds of this and that, it can pay off. If this is something you are considering for a couple years, then it is well worth the hassle. If you are going for a little work holiday for a couple months to a year, I would reconsider.
I worked as a dive instructor in Phuket for 9 months. My WP was only valid for Phuket and in order to work anywhere else I would need to obtain different documents. I'm not familiar with English teacher visas or standards, but also within my WP there did not seem to be any real standards from one lawyer/accountant to another so I never knew if i was getting the "deal" or getting ripped off. Not to mention I can't read all the thai documents, so who knows what I was signing.
I hope this doesn't deter you, however it is the cold hard truth that you do need to consider. Also what line of work you are doing and for how long. You also need to research various people who can set it up for you to ensure you aren't over spending or getting ripped off. As Curt mentioned, Thailand is fairly strict on who they bring into the country as they believe if a local can do a job, then they should hire them first. Although if their is a position that can not be fulfilled by a local, then it will be offered to a farang.