I think my biggest complaint is when you repair or replace something with successful results, and someone swoops in to say, "You should've done X instead," based on their experience with what worked for them and their buddies, or their preference in equipment.
I won't use any examples from here, but when I was frequenting Jeeps Unlimited, and swapped my narrowtrac axles for widetracs, I had all sorts of "un-asked-for advice" on how I should've instead upgraded to Dana 44s with different gears and lockers than I had - because factory widetrac axles are junk and would be lucky to survive street driving. Nevermind that fact that I'd scored the axles for $400 each (completely loaded with Detroit Lockers, 4.10 gears, one-piece axles, trussed, etc.) from one of the very same guys who was swapping to full-size axles and HUMVEE wheels (he liked to bash his Jeep every weekend).
There's always going to be comparisons of different things because there are so many ways to do the things we need to do (example: MSD vs. Pertronix vs. Duraspark vs. points, et al)... but telling someone they made a bad choice because you prefer something else is what I'm talking about. When I have something in mind, I usually run it by the group (because I know 'somebody' has had experience with Brand X) looking for affirmation, potential pitfalls, and 'lessons learned' type things... not, "you should've done this instead because I like it better (or it costs more, or it's a better name-brand, etc.)."
Fortunately, there's not very much of the "mine's bigger and I know someone with 2"-syndrome among this group.