That's your opinion, but the way I see it...
Graphics - Not quite as good as GT4, but hardly mediocre, and some of the best I've seen on the PS2. I'd rate Enthusia above GT4 in special effects.
Sound - The engine sounds are unique across the entire car selection, and far superior to any Gran Turismo game. You can tell they used actual recordings for the engines, and the other sounds change realistically as you upgrade your car. Unfortunately, certain sounds (eg. turbo spool, blow-off valve, brake squeal) all sound the same, the looping is blatantly noticeable on some engine sounds, and the music is hit-or-miss even if you like "videogame music."
Limited tracks & cars - Compared to GT4, sure. But you could say the same thing about pretty much every game that isn't GT4. Enthusia still has an above-average amount to choose from, and is roughly on par with Forza.
No damage or online multiplayer - The only reason this is a factor is because GT4 was forgiven for those same flaws -- because it's Gran Turismo.
Confusing gameplay - In Enthusia Life mode, certainly. I don't know how anyone could be confused by the rest of the game, except for perhaps clinging to the misguided notion that Driving Revolution is strictly intended to teach you the racing line.
There's no doubt that Enthusia could have been better in all of the above areas, and I'd agree that those who dislike the game tend to complain about those things (almost always making a reference to GT4), but to blame the game's overall quality for its failure is unfair, in my opinion. Simply put, Enthusia failed because it was too realistic, and because it wasn't GT4.