- "You know, Target sells something that [does that]/[does almost exactly that]." This is more common in meatspace than on the web, I think, because the web audience self-selects for people who understand that the joy is in the making, not the having. And, frequently, the "almost" is where the bugaboo lies- I want EXACTLY that, not ALMOST exactly that.
- "Arduino is for losers." Or some variation thereon. Arduino, as a platform, is a lot like Python: best used when development time is the most precious commodity of all, more precious than CPU cycles, money, or cachet. People who get that, use it. People who don't get that spend a lot of time trolling comment threads but never actually make anything.
- "Using a microcontroller/PC parallel port/Arduino for that was total overkill." Yes, I know, you could have built that project with four 7400 series logic chips, three capacitors, a resistor, a piece of perfboard, and an inflatable duck, but A. I didn't have those on hand, B. designing that circuit, wiring all that crap together and troubleshooting it would have taken more time than I wanted to spend on the project, and C. I made it and you didn't, so shut up.
- "I/my cousin/Nikola Tesla/Jeri Ellsworth already did that better." That's great, really. If my "project" page is just a detailed description of my assembly of someone else's kit, I probably deserve to be chided (or at least ignored). But if not, then the odds are that I either did it ignorant of the prior art (which means my work should be judged on its own merit, not against someone else's) or I think I have something to add on the topic (in which case, you probably didn't read the post to see what I added before commenting, and shame on you for that) or you/your cousin/Nikola Tesla/Jeri Ellsworth didn't post it on the web or adequately document it (meaning, my documentation of the project adds to the richness of the human experience, or something).
- "You have too much time on your hands." Another troll comment from someone who thinks they're hot stuff but never actually does anything. What, pray tell, SHOULD I be doing with the tiny little amount of spare time I carve out of parenting, working, housekeeping, commuting, and sleeping? I MIGHT tolerate this from someone who doesn't even OWN a television (because if you own one, I guarantee you spend more time watching it than you think), but since that's only something like 1.5% of the population and that 1.5% is too busy being productive to level such a shallow criticism, anything I'd suggest to this commenter would be inappropriate for a public venue.
So, as your mother should've taught you, if you don't have anything nice or constructive to say, STFU.
Last thought- this is a major reason hackerspaces are awesome. Most of these comments are easy in the faceless netvoid but you'd NEVER trot them out face-to-face. Plus, by their very nature, hackerspaces attract the people for whom the doing is the most important part; for them, results speak louder than words.