Quick disclaimer: I really liked Monte and was sad to hear that he has left. However, unlike a large part of these forums, i dont see it as a death knell or bad omen for the edition as I think the current team is solid with or without him, it just would have been nice to have him there.
As to his L&L columns, I felt that the polls at the end were often times heavily slanted at getting a particular answer. It seemed like for whatever question/situation that was presented for the poll, there was a kind of pre-selected favourite choice in the way the options were worded. I know alot of people were pretty vocal about that.
The passive perception thing was mentioned above, but I also think that was made worse by that being his first L&L column. Also, someone else mentioned that he suggested different ability scores based on gender that incited a big enough reaction that it was retracted? I didnt see that one myself, but thats what I read elsewhere on this forum.
I personally liked his L&L columns overall. Although he had a big hand in 3e (I know alot of people critisize/praise him for his preference of that edition), I felt like he was moving beyond the scope of 3e, and I still think thats how it is being handled, even without him. I also think that the rest of the team has also been very good about moving it forward beyond 3e, and 4e. They are really taking a hard look at the older editions like BECMI, 1e and 2e as well, which I think is a factor that often gets overlooked by most caught up in the 3.X vs 4e mentality.
Again, its unfortunate that they lost a great writer like Monte, but lets not write off Mike Mearls and the rest of the guys working on DDN. Im a little saddened to see a large amount of ENworld people so eager put the rest of the DDN team down like that because Monte left. Its just as bad as those who are making rude comments about Monte's departure. I know Mike Mearls has always been a big fan of 3.X as well as the older editions, and he has been very vocal about the vision for 5e, so lets not jump to conclusions. This is what the playtest is going to be for, Once you've seen the hard rules and can make sound judgement, that is the time to let WotC know how you feel.
Sorry for the rant, but after reading so many doom/gloom posts from people who are just having a knee jerk reaction, I just had to vent a little bit myself.