Thanks for the compliment Haroon, although I don't feel worthy. I have released only one mission before but have hundreds of attempts that never met my final quality standards and so they got scrapped. But through it all, I have enjoyed myself and the ability to learn my way through the editor and game. I still may never produce something that others enjoy but atleast I am having fun trying. I have no fear borrowing ideas from other mission authors to use in my own stuff(with proper credits of course) and so our missions do not have to be entirely original to be fun. Afterall, Fun is what it is all about. Not complexity or originality.
Its true that the landscape has changed as well as the mission making process(to an extent). But then, folks are always submitting new models and islands for use to the community and so the landscape does not have to be the same nor the story. Some try and recreate scenarios that they see in movies and some politics while others just follow the standard mission ops platform.
You can not compete with the resources that the BIS team has at their disposal so don't let that fact discourage you from making missions. I have often thought the same as you when pondering mission objectives. It seems that they have all been done before. But something to remember is that even in reality, those military operations have been done before time and again. The difference being that each occurance presented the operation commanders with new terrain and new enemy fighting styles to adapt those tactics to.
Ask yourself this, How many times have you seen OFP,ARMA,ARMA2 missions that are similar but in different areas, be fun to play?
Pick a mission type, then pick a location(whether that is a standard island or an addon) then try and build that mission type into the location using the terrain to the enemies point advantage. Put yourself in the seat of the enemy commanders and construct your base/defenses that would best ward off assaults. Try and think of every possible tactic that would overcome your defense and then implement countermeasures. Then leave it to the players to find the weaknesses. When they do, and they will, you will then have another tactic to consider defense against when you build your next mssion.
As a mission editor, you can not properly test your own mission. Because you already know all of the secrets. But others will not know those secrets and so something that seems ordinary to you may not be so to others.
Arma2 beta boards are being flooded with all kinds of missions which can cause your project to become lost in the boards and under appreciated. Don't get caught up in competition. Do it because you enjoy it and don't worry about if its been done before.
Some last words of encouragement? Don't let the awesome mission editing skills of others get you down. I have seen some truly simple missions from new editors but enjoyed them just as much for their simplicity and ability to allow me to just play something new. In the end, we're all in this addiction together, we don't mind helping you hash out details. Just ask.