when writing or reviewing homebrew, what is the most important thing you look for? What is it that you want to prioritize? I just think it would be interesting to see what people think

If the option is clearly unbalanced or the language is bizarre, it's usually not worth reading further. Flavor and fun is going to be in the eye of the beholder, and nothing is wrong with reflavoring or tweaking existing mechanics.

I think that balance and interesting mechanics are the most important part of a creation. Well written flavor is excellent, but everything can be reflavored and you don't want flavor to lock you into a specific niche. 

I found it very hard to choose one option over another as most important; in the end I settled for how fun it is mechanically.

Whether something is fun to roleplay is down to the player, DM and group more than a class/race/whatever itself, if it's fun mechanically then it should naturally enable fun RP. I would also say that to be fun mechanically something should be balanced; being powerful might be fun for a while, but if other players are weaker then either they start to feel weaker, or your DM has to scramble to make up the difference somehow, and I don't think that's super fun. It can happen with some official (sub-)classes depending upon your campaign anyway, but the official content, despite its flaws, is generally quite well balanced overall, so in homebrew terms I'm thinking of something that goes beyond that in a bad way.

That's my thinking anyway, as for me at least, balance and fun mechanically and RP wise are the three most important together; consistent ruling and formatting isn't crucial but I think the best homebrew tries to be consistent with the main rules, and follow the wording and patterns of similar features where possible, as it just makes them easier to play and for everyone at the table to understand.

Novelty in homebrew is an odd thing to say is less important, but I think it's true; you can have a blast playing a homebrew sub-class that's just a mash-up of two other sub-classes, or an item that's disappointing in the base game that's tweaked to just be a bit stronger and so-on. I love original homebrew, but the stuff I actually use the most are more like functional tweaks, or filling gaps (e.g- more acid spells for an acid sorcerer). So while novelty is great, it's not critical to good homebrew.

One thing that's a pet peeve of mine on homebrew that isn't covered here, and which I consider to be by far the single most important thing for any homebrew designer to do, is to make sure your descriptions tell the player everything about how your item works; I've seen numerous homebrews that sound like they could be fun, but where I would have to add them to a character to find out how they actually work. I know it's a pain to keep your descriptions clear, concise and consistent, but if someone can't understand the entirety of your homebrew from the details page, then that's not good, and means people (like me at least) will just ignore it; probably the most common example is on sub-classes where authors have forgotten to note in descriptions which level a class ability is available at, requiring players to work it out from other sub-classes.