I usually see the phrase "slow burn" used for art that is boring at first and takes a while to really impact or hook people. Dennou Coil starts as a pretty alright anime and then gets better... and then it gets even better.

I can describe the premise pretty easily: we follow a group of elementary schoolers (about 9-10 years old, if I remember correctly) in a small Japanese town, in a near future where AR glasses are the new phones. Everyone has these glasses at all times. They're used for stuff like maps, internet browsing, phone calls, social media, etc. People have 3D cyber-pets that can only be seen with the glasses, and use glitches in this cyberspace to get weird items and do illegal exploits.
There's shady stuff going on with the cyberspace, and it introduces concepts that would be dystopian as fuck in real life, but even if the show acknowledges that they are there, it's not about that.

Dennou Coil is a ghost show masking as a sci-fi show. If you judge it for its technological realism, you'll be very frustrated. But if you accept the sci-fi aspect as part of its tone and themes, which work around a more traditional story about what would otherwise be spirits and mystical shit going on, it'll make a lot more sense.
There's places with eerie fog and shiny trinkets that can be used for creating magical seals, but they're called "obsolete space" (since they're an outdated version of their cyberspace), "metabugs" (explained as exploits of buggy code) and "metatags" (which are some sort of arbitrary code execution). People are safe from certain hostile entities in their houses and at shrines... because of their local regulations, which don't allow them to enter these spaces.

The general mood of the show swings between whimsical and very serious, which I appreciate a lot. I find that I care about the characters a lot more when I can also see them having fun and being silly. It gets more and more emotional towards the end, and the early silliness makes it more impactful.

Watch it, if you have the space for a show! I recommend it.