It took some elbow grease to make it through the single small nest, but after beating it, I don't feel especially inclined to continue playing. Even if there are more enemy types I haven't encountered yet, pushing through the early game (as I'll explain later) doesn't feel like a fun enough task to justify being a completionist here.
I see why this is one of the classics. The gameplay is simple, the slow-but-powerful swing attack is easy to grasp, satisfying, and requires strategy, and despite enemies having almost no AI, a lot of strategy emerges from deciding if and when to engage different enemies.
The biggest weakness in my opinion is that you spend too much time in early game, and for a game as punishing/difficult as this, it can dampen the game experience quite a bit. You spend a LOT of time pushing through the first 50 or so enemies repeatedly, especially because being killed by charging enemies with almost no warning (due to them being off-screen) happens easily and often. I get that area denial and controlling the center of the map is a central part of the gameplay, but you don't need instant death to hammer this point into the player's head, and doing that ultimately produces a frustrating experience where players suffer a cheap death, which makes them want to speed through the easy part of the game to reach the end, only to recklessly die again. If you could restart smaller sections of the game in which you could begin with the spawn rates of late game, it would have made this game much more satisfying as the player would be confronted with challenging content all the time, rather than constantly dying and having to repeat boring sections.
The bells and whistles on this game (different modes, powerups, bestiary, etc.) are all nice additions and add a sense of professionalism to the final product, but I don't think these change a whole lot. For the most part, the good parts of the gameplay (strategic depth) are present in every mode, and that's the important part. The issue is that the weaknesses (redundant early game) also affect the entire experience.