I'd have to go with the simplicity. A lot of toolsets I've used feel very... backend-ish. Not that itch doesn't have this, but imagine if itch's butler tool was the only way to manage your games up here. That's what a lot of sites do. itch on the other hand is user-friendly for both developers and gamers! The latest additions to the page designer are also great, like being able to set the alpha level for the second background color, or choose any Google Font.
There are two things itch needs, and they're both related to financially supporting developers:
- A cart. Needing to purchase one game at a time may be too tedious for some gamers, especially come sales time, when they can just, say, go around Steam and click "Add to Cart" on a dozen different games before purchasing.
- Some new ways to support developers. Something like a microtransaction system comes to mind, where you can buy 100 points for $20 and spend them all around the site. I think gamers on the site would be a lot more willing to buy some games (or even comics and such) up here if they could get them from a lump sum. Plus there could be ways to earn points, such as rating games, being good community members, etc.
But overall itch.io is easily my favorite site for posting games due to its simplicity, fun, and design-focus.