To whom it may concern,
I am very sorry to inform you that I have decided to hand in my resignation as of ......(December 31, 2007).
When I first came to your office, I was full of energy and also passionate to do my job efficiently. But as time went by, I have come to believe that I cannot realize my full potential in your firm.
--> Sorry, I kind of changed all of the latter: You should try to avoid accusing people directly of a bad performance. The company's boss might take this as a personal insult and kick you out instead of accepting your resignation (if he is allowed to do that) or at least he might not give you a very nice reference letter for your next job. Though your task was to "criticize the bureaucratic inertia of the office" you should try to do this in a more subtle way. I do not say that the following excerpt is better as I also am not an English native speaker, but I gave it a try:
Unfortunately, I have not been able to fulfill my job in a way that would show my real talent. There have been many occasions where I felt that my enthusiasm and willingness to do my best were slowed down by my surroundings. Since the department I work at in your firm is presented as a 'team', the department's performance and attitude towards work also reflects on my personal performance and attitude, me being a member of this team; and I am convinced that the two don't always go conform.
I, again, regret having to tell you that after a very long time of concideration I, because of the reasons mentioned above, have come to the conclusion that I can no longer work for your company. I believe I have to give my career another chance and try to find a job where I can better unfold my potential.
I am nevertheless grateful for all the experience I have been able to gather at your company and hope that you will accept my resignation.
--> "a bunch of people": You don't want to use a phrase like this in a formal letter. That is spoken language only.
Student or Learner