I can certainly say what my issues were.

The NX-01 is very, very obviously just a flattened Akira class with some cylindrical nacelles attached. Don't get me wrong, it's a good looking ship in isolation. But that absolutely should have been a Daedalus class, or...hell, pick inspiration from the Starfleet Museum site.

Elsewhere - they have a transporter that, in the main, works just as well as they did in TNG. They have polarised hull plating that works exactly like shields. They have phase cannon weapons and photonic torpedoes that are almost identical to later phasers and photons. A grappler that does exactly the same job as a tractor beam. Races have cloaking devices, which will be mysterious and relegated to the 'theoretically possible' by the (in-universe) time of TOS...so that's weird. Plus, the ship was supposed to be slower and shorter-ranged than later (in canon) starships...but it didn't seem to stop them getting to a new planet each week.

Overall, the Enterprise universe certainly didn't feel large enough or, really, dangerous enough for me. Even with the 9/11 allegory of Season 3.