UI toolkits - Seems that all the highest quality UI toolkits (Ant, Material, Fabric/Fluent, etc) have way too much whitespace. It works for things that work well in "cards" but the low information-density is killing me personally. I think more developers would choose high-density UI toolkits if one was released by a major dev team and could be relied on to be maintained. Maybe Photon/Protocol could eventually fill this gap, I like them a lot more overall but they don't seem nearly as well-packaged for easy use, or at least don't seem to have a complete kit available at
https://www.adobe.com/products/xd/resources.html or anywhere else really.
Walled gardens - not being able to build things that integrate multiple services. Instagram live videos, combining amazon and walmart listings, google places "here now", or unified client for iOS/SMS/Discord/Slack/Teams are all off limits to ambitious developers.
Centralization - Seems like home email servers are increasingly being ban-hammered by the major providers like Gmail, ProofPoint, etc so you have to have your mail sent by a hosted server. CloudFlare is also seemingly becoming a single point of denial/failure. Chromium can now effectively dictate W3C standards because whatever they implement instantly becomes the ground truth for web dev.
DNS/BGP - Sorely in need of a major update.
IoT - It's shockingly difficult to find IoT devices which can compute locally/privately/on-the-edge. Everything sends all the data back home and stores/processes it there.
Tracking - If websites insist on having an OAuth or email verified signup, I'd like my browser to quickly and quietly create a new profile for every domain, or even every visit to that domain. And otherwise keep all tracking/fingerprinting down to effectively zero. I'm really tired of being tracked everywhere all the time.