Honestly? We can’t.
“I think when you’re putting forward a point of view there’s two ways of doing it, one is to say something positive and the other is to criticise the negative. It’s a bit like the sculptor who was asked ‘How do you sculpt an elephant?’, and just said ‘I just take a block of marble and then knock away all the bits that don’t look like an elephant.’ In other words, by criticising certain things you help to define, by implication, what is sensible.”
— John Cleese
You don’t get the elephant by pointing out the fine qualities of the block. That said, there’s a way to do this critique badly and a way to do it well. Doing it badly looks like this:
Doing it well, meanwhile, is in part about how you construct your critique and in part about choosing what you critique and where. Some people are not looking for critique; they want to express themselves and aren’t interested in refining their work. Testing the waters and, in essence, asking for permission to critique is par for the course in some online communities I’ve known. Building relationships that allow for friendly criticism and build a certain thick-skinnedness helps avoid that slightly artificial case of affairs – banter in a community is good, it means you can hear something that might reflect badly on you and respond with “par for the course rather” than “WELL I NEVER DID YOU HEAR WHAT HE SAID ABOUT OUR SHARON?!” Part of doing that, I think, is extending your ‘universe of your own eyes and ears’, the blogosphere you perceive and interact with, beyond people who agree with your perspective, ideas and style.
I’m not saying go out and find somewhere wildly different to operate; I’m saying ‘look for the boundaries, and operate there’. I’m not part of the Old School Revival or Renaissance or whatever – I don’t feel terribly beholden to D&D in any edition and I personally incline more toward armchair theatre than to mechanics and exploration – but I hang out on OSR blogs that produce discussions I’m interested in and that have some bearing on what I do with RPGs regardless, because it’s that boundary between someone else’s priorities and mine that produces interesting discourse that we could all actually learn something from. Same reason I discuss Games Workshop games even though I barely play them any more – because there are differences of opinion to be had there and difference is what refines an opinion through testing and application.
You need friction to start a fire. Whether that’s a campfire suitable for putting a brew over and staying warm through the long cold night or a raging inferno that devours the forest wholesale is down to the control skills of the people involved, which brings us back to that first paragraph. However, the trouble with fire’s that it burns if you get too close to it. I’m afraid that even the very politest criticism is going to ‘eat goodwill’, as you put it. It’s an unfortunate side effect, but unavoidable – nobody can separate themselves entirely from something they’ve invested their energy and time in. Alas, for a work to be improved by its criticism, or judged honestly in its light, all parties involved need to suck that up and live with it. The author is dead and the work will be judged on what it is, not the process of its making. There’s a difference between a spark and a flame, and I think too much stagnation in communities leads to the former being seen as the latter more often than not.
I learned this the hard way – my first contact with the Internet was with fanfiction, a territory full of small communities of people who can’t get away from each other because that common interest takes them to the same places, full of overinvestments of the author in their work and inabilities to seperate Who I Am from What I Do. I wasn’t kind in those days; I was young(er) and stupid(er) and full of conviction regarding my own talent and advice for people who didn’t need it, didn’t want it and probably realised it for the collection of obvious truisms and superior wankery that it was – but I didn’t stop giving advice and trying to improve people’s work.
I’d much rather point out something that’s stopping a work achieving all it’s capable of than highlight the bits that are fine. The good things about a work are not the parts that need attention from anyone but an advertiser; it’s the flaws that are holding it back. Praise has a place in criticism, but its place is to couch and contain the critique, not replace it.
Criticism is necessary in a creative community because it helps make things better, and it’s damaging when someone makes assumptions about whether someone else wants their things to be better or not. Assuming that someone wants to improve their craft, it’s practically the responsibility of a decent community member/friend/human being to help them do that, and telling them only about what they’re best at is not going to help with that.