I've noticed Hallow alts have become very popular, with Corruption alts starting to pop up more as well. I've been seeing some common mistakes with a lot of alternate biomes:
Reskins: If your biome is possible, or nearly possible, through texturepack, then your biome isn't unique enough to be an alternate. The idea becomes more of a texturepack description than an actual new idea for an alternate biome. While your biome may look different, it should also feel different, but work similarly.
Enemies: Of course, not every enemy must be completely unique, such as slimes, though it'd do you well to have a fancier slime than just another different colored one, like ice slimes. With your other enemies, they should be as unique as possible. Give us a reason to want this biome besides graphics!
Sprites: You do not need them, however it is strongly recommended. This goes for most suggestions, but most especially biome threads! We want to see what your biome looks like. I cannot stress how important sprites can be if you're trying to win over support. You do not need sprites for everything, just give enough so we can feel and see your biome, which is something that isn't as easily done by words alone.
Posting an incomplete idea: Again, for most threads but mostly biome threads, you should wait off until you have enough for your idea to be presentable. If there isn't enough to interest someone, they'll leave the thread and may never return, even if you add in more content. It's best to wait off until you have more to show. Don't rush it! "I was rushed, I had things to do, I didn't have the time!" are not good excuses! Just save your work and continue it later. It took me several hours over several days for my Confection, and I certainly did not do that all in one sitting!
And some bad habits by those commenting on biome threads:
Comparing to other suggestions: Biomes should be judged independently from others. Just because you like one more doesn't mean you can't support another! This is mainly for alternate Hallow ideas that are paired with the Crimson. Zoomo does not own the Crimson! A biome should not be beat down if the creator puts it up against Crimson, if you like the Cyber. Liking another alternate Hallow paired with the Crimson does not necessarily make you anti-Cyber. We're not even sure if alternate Hallows will ever be a thing, but if so, who says biomes will be locked together? It is very possible that if the devs implemented the Cyber, we could have Corruption/Cyber worlds. Soul eaters vs Soulless, right? If this new Hallow alt for the Crimson came out before the Cyber, would you bash on the Cyber just the same? I've seen "This is just a rip off of the cyber/contagion" before, as well as "There's the Cyber, so this idea is redundant." That's rude and helps no one. Suggestions are not official content, so don't treat them as such.
Simply saying you support: While support is greatly appreciated, it's also important to point out what you like/dislike and what could be improved on!
....yeah, that's all I got I think.