I see a lot of folks have brought this up, and I guess I'll echo it because I full-heartedly agree with this notion: Kanon was a good stepping stone for Kyoto towards perfecting the Visual Novel forumula that eventually led to the masterpiece that is Clannad and After Story. Unfortunately for Kanon though, it means the anime formula wasn't perfected yet when it came out and was plagued by a lot of the issues that VN adapters struggled with for years until they finally nailed it with Clannad.
There are several kinks in Kanon that really hold it back. To quickly disect it's core issues that I'll elaborate on section by section: the MC is too blank, it's supporting cast barely contributes or sticks around, the main cast are heavily segregated by arc, and the setting was just... bland.
The main character (Yuuichi) was most definitely a protoype for Tomoya from Clannad. Like Tomoya he was funny and liked to mess with people, but he was made extremely bland and generic - with his memories blank and none of his backstory mattering outside of the ones he gradually recovers of the girls in the town he's come back to. Tomoya had a solidified, notable backstory that defined his personality and drove his actions. He was more of his own character that we followed along and learned more about - and made the relationships he was building upon feel more impactful. In stark contrast, Yuuichi on the other hand was a run-of-the-mill harem protagonist that the audience could project onto due to being less of his own character and being design as a mostly blank canvas. It made me care much more about Tomoya, and far less about Yuuichi.
The supporting cast was far less notable or as interesting as Clannad. The best friend for example (who also shares Sunohara's VA) didn't really do anything and was kinda just a background character whose only trait was being a simp of Kaori. The girls also didn't really contribute anything notable towards the story before or after their arcs - they were heavily segregated and didn't build off each other. The other less notable extra characters didn't contribute or have overly complex emotions or involvement that gave Clannad's extensive cast more life to them.
This last one is more a minor annoyance, but it grew on me more that I feel the need to speak up on it: The setting was boring. I didn't like how all of this took place in the span of 1 winter. We never saw any expansion of the setting and it started to become much less interesting after awhile. A small snowcovered town that never changes and we barely see anything outside the sidewalk route to school, the school itself, the home of the MC's cousin and aunt, and the woods can only remain interesting for so long. A 12 episode anime it could pass, but for 24: no.
The OP was also a bit of a give and take. It revealed what route the anime was gonna go in and which girl was gonna "win" by the attention given in the OP. While Clannad did somewhat do the same, it was far more apparent in Kanon's. The song was far more emotional compared to Clannad's first season though (but it holds no candle to After Story's).
Kanon had potential and there are things I liked about it. But unfortunately it was not fully lived up to and had a bit too many things holding it back to put it on the same shelf as Clannad. Timing is a big thing, and due to formula of adapting VN's into anime format not being perfected yet Kanon's end product can be considered: borderline experimental.
I doubt we'll ever see Kanon revisted (outside of that Kagi-nado series, which is basically Isekai-Quartet for feelsy animes like Kanon, Clannad and Little Busters) given this is already a remake and there have been other far more successful productions made by Kyoto. But at least it can be credited with helping work out the then-unsolved kinks to pave the way for Clannad. Good but not great.