Terraria is a great game, but some aspects of it are implemented very poorly. Some of them, in my opinion, are:
Celestial Pillars mechanics and weapons. I'm not a big fan of grinding a total of 400 enemies just because it's necessary to process further. This is an absolutely not fun thing, same as most terraria invasions, because their mechanics are simply to kill N similar enemies. I like the way the Pumpkin/Frost Moon is designed, because the difficulty level increases depending on you. But most other invasions come down to just killing 150 enemies and trying not to get killed yourself by them and their projectiles flying chaotically across the screen. If we talk about the pillars, they don't really feel like something should be in endgame, it just seems that there are parasites in your world that you need to get rid of.
In turn, their weapons are the most incomprehensible thing for me. For example, you collected different swords throughout the entire game, from the early Pre-Hardmode to Post-Plantera, and finally you create a Terra-Blade. You defeat 2-4 bosses with it and it goes to trash bin just because you got weapon you would get anyway. It is unfair having weapons "given for free" to be stronger than the weapons you have to complete a challenge for, it kills the sence of latest.
Master mode. This should be, like, the last challenge (apart from the Legendary difficulty), but now it looks like No-Hit mode. It changes very few game mechanics. Look at Expert, it changes every boss AI so that it has fewer vulnerabilities, adding mechanics that makes the boss feel like a boss, like several new attacks such as EoC dash or Skeletron skulls. If you look at the Master, it's basically just stat boosts. You want to get a challenge when you compelte Expert, but you just get the same Expert with thicker and more damaging enemies. I like how Re-Logic implemented Master mode rewards (it doesn't give any advantages, but it makes it clear to everyone that you are cool, this is the best solution for rewards at the highest difficulty), but they forgot about the risk and, as I said, made an Expert with better stats.
I don't think devs will do anything about it, since it's all already caught on in the game essence, but I expressed my opinion. What about you?