To some extent, those that are summarily decrying the "actions of reddit" are judging with the benefit of hindsight. Here's my take:
I believe the people who participated in those threads on reddit/4chan/etc. we're actually, truly, trying to help. (I'm not one of them, but I watched much of this unfold).
Some folks spent hours pouring over any publicly-available images they could find to see if anything stuck out. They looked for patterns, looked for backpacks, tried to match up people to different pictures, etc.
Had this gone the other way -- had one of those pictures been of the suspects and had one of the "Internet sleuths" circled one of the brothers instead of a random innocent person with a bulging backpack, we'd be having a very different discussion about what heros the crowd can be and of the virtues of crowd sourcing in the modern age, etc. and frankly, had that happened our praise would be just as misplaced. Shoot a machine gun randomly around you and kill a bad guy, you're a hero. Kill a good guy, and you're the enemy. Still doesn't meant it was a good idea to shoot like that :)
So now they got it wrong. And that's going to happen. And now we're vilifying everyone.
Here's where I think the line should have been drawn, and the lesson that we all should learn from this: I think, actually, had folks just stopped at the "this seems suspicious" stage and sent that info to the guys who really have details (the FBI), that actually could have been helpful. I know if I were an intelligence analyst, I'd certainly find it even slightly helpful for someone to do even a cursory pre-examination for me. Sure, the kids don't know all the real techniques or have all the real tools, but even 10 minutes saved in picking out some potential people to cross off the list can be helpful.
However -- the line should have been drawn there. Find the "interesting" items, alert the authorities, and let the take it from there. No names. No accusations. And certainly no front-page-of-the-New-York-Post publishing (and that blame lies squarely with the fine folks at the NYP, not reddit).
It boils down to knowing the limits of your abilities. The crowd is good at scanning a lot of data and raising its hand at things that stick out.
The crowd is not good at being an actual intelligence analyst.