With so many highlights in the year, it is very difficult to choose 1.
If I were to choose 2 most significant ones, they will be ISC2010 offline event at Techfest and beginning of Monthly tests.

I agree with you Deb.

The most significant was the start of Monthly Tests with participation level increasing gradually. Also, the offline event at Techfest was a success for LMI.

Another important milestone in the LMI history is the launch of sudoku and puzzle ratings which is certainly going to increase the competitive spirit in the puzzling world.

Mastermind twins is the test which still comes top of the mind, when I think of LMI tests. So, I guess the LMI test culture which was brought about in May 2010, has to be the highlight of the year.

Good point Neeraj, I missed it.

Yes, Mastermind twins introduced 2 key things at LMI tests
1) Naming a test - This is one thing that I personally like most. It is very difficult to recall what was mock16 or who wrote puzzles for mock16 for example. But it is easy to recall a test when it has a name.
2) Theme based / Related puzzles - This is something we still follow today, and may be we can follow as long as players enjoy!

I will vote for starting of monthly tests. Another I will agree with Neeraj regarding the starting of puzzles and Sudoku rating. Next one is going to different colleges for IPC and ISC 2011. Even though this event is going to be in 2011 but this was planned in 2010.