I confess I’ve avoided this corner of the forum because my first answer is: “just one?!” so I’m late to this party!
Don’t get me wrong, I dearly love Folksy and all that is stands for, but there’s a lot of things that really need some serious improving. I think any ‘one’ change would have to be aimed towards customers: making it easier, better and also getting them here in the first place. So that has to be priority if we’re talking about ‘one’ fix, rather than making things more comfortable or smooth for us as sellers (to my mind).
Search: search, search, search - or at least an easier way of browsing (better categories, more ways of filtering) - or ideally: both! The search function seems to have fallen over and it’s so hard to find anything on here as a shopper (particularly for multi-word long-tail searches - which are the exact ones that should be spot on because those customers are the ones that want to buy and know exactly what they’re looking for)! This has got to be the number one fix - the site has to work properly (for customers) before anything else is tackled. Not every shopper knows what they are searching for, so they’ll use the categories (so the categories need to be really good - and once you’re in a section you need to be able to filter - see NOTHS for example) - but those that do have an idea will use the search function, and that is letting the site down at the moment as we know. If the search function is seriously hard to sort out (as Folksy seem to be saying), then can we not in the interim/instead have MUCH better categories and filters and encourage shoppers to use those by having them spot on?
Photography: it has got to be better, this isn’t Ebay - the dodgy photos taken on bedspreads or with the telly or washing in the background don’t cut it: in fact I seriously believe they harm the site BIG time! I’m not suggesting it has to be professionally shot photos of course, but when I compare my shopping experience with when I shop on Etsy, this site can feel like a strange mix of professional and carboot. Sorry if I’m offending anyone here, but some images are truly awful and it doesn’t matter how lovely an item is, if the photography is dire people won’t buy it (or will expect it to be cheap) and it reflects on everyone here and their impression of the site. I think the bad photography also makes the site feel fuddy-duddy - rather than fresh, modern and stylish.
Marketing and Promotion: No, the site doesn’t have the budget of the big guys, and yes, I know a lot is done already. But it could be better, and not necessarily expensive. Etsy for example, I often see where they’ve got a celebrity or influencer picking items (or in some cases, actually creating a line to sell on the site) - which creates a buzz that this is a stylish and interesting place to shop. Colabs with bloggers, getting into the press, being included when something like ‘This Morning’ (or whoever) does a feature on what to buy your husband this Christmas (or whatever)! Even little things like having discount offers from time to time or being included on the cashback sites like Quidco (I know this might be seen as a bit low-rent, but I personally have bought something from a seller’s Etsy shop rather than their Folksy shop because I know I can get 5% back - in this day and age, every little helps!) The site has such a great USP that should in theory overshadow Etsy here in the UK, and that needs to be shouted from the rooftops! We’re not a site that allows fakes or mass-production (E, E, NOTHS), and we’re a site that’s true to the values that it’s always had - and that should bring shoppers who want to buy mindfully and creatively knocking at our door. Us as sellers, individually telling our friends and hairdressers about the site isn’t enough. Us as sellers, creating our Folksy Friday boards and doing what we can on social media, isn’t enough. We almost need to do more pooling of ideas/resources or something. But Folksy need to be driving this from the front. Somebody (I think @JudyAdams) suggested vetting shops before they’re allowed to join: yes! this! (And that would also give Folksy an opportunity to weed out the dodgy ‘excuse the dog/husband/washing in the background’ type photography). Make it curated, make it so that it’s featuring the best and therefore THE site to come for shoppers. The things that make Etsy weaker (the mass produced, the fakes, the fact anyone can join and list their dodgy-doings, the sheer size of it) could all be things that Folksy become known as being awesome at. Oh, and better SEO for the site - make sure it’s always on page one for more search results.
Personally, I also hate the new photo layout on listings - it looks clunky and it feels clunky (scroll down to see more pictures, pick one, scroll back up the screen to look at it…) - it’s bonkers! It also feels as though making them bigger has ruined the quality too somehow and quite often they look too low-res? The site refresh should make things better, not more clunky.
I do find myself wondering about the ‘no one has heard of Folksy’ thing - how, how can that be? It’s been around for years, it’s established, so what’s going on? I honestly think people do find the site but they get overwhelmed by the easy choice of ‘go to Etsy’ - they might come here, have a brief scroll around, and then leave for a site they feel familiar with (and can actually find things on) and which feels more modern - and which is a site they’re constantly being reminded about by promotion so stays in their mind. I think the key thing here is the site needs investment (time, experience and financial) to bring it up. It either wallows and bumps along, or it competes. 
At the end of the day, more sales is better for us but also better for Folksy - more to spend on promotion, more to spend on staff to help with promotion!
Oh, and if you’re still reading my essay (sorry), I would like to thank Folksy for asking us and being open to our ideas 