Personally, I really enjoy small vanilla survival servers. Really makes everyone close to eachother.

I'm going to try and avoid subjective and speak from experience, logic and observations. That's not to say an attempted objective take is automatically correct, so take what you will. It's not quite what you were looking for, but who knows.

What makes a good Minecraft server or Network?
A disclaimer, a server doesn't actually have to be good to be popular. It seems to me that half the time, it's not a matter of how well you made it, but how lucky you are in getting that first jumpstart of players. Keep them going and you can grow from there, build a community dynamic, and get big off an otherwise pretty objectively bad server as far as the backend goes.

So, while I've always obsessed over the quality of a server's design, I have to conclude that it's secondary compared to the ability to network and encourage people to make your place the one to spend time on. Keep them interested, keep them engaged, and this can be as simple as being the owner who is there more often than not. You're not going to get far not slaving away ingame as a presence unless you get more luck than you can hope to predict for.

By networking I do not mean advertising - that's a good start, but it can be very hit or miss. I imagine that PMC will be your best bet as an advertising place. No, by networking I mean to get into the community, know people, make friends and factions, and be able to say 'hey look check this out' when you've got a server going.

As far as good on the backend, I'd say polish is good. Configuring things to be consistent and blending into each other as compared to doing what literally everyone else can do and often does - slapping a bunch of fancy plugins in a folder and calling it a server. That's a way that does work, but it's not one to bank on, and I think more importantly, you don't learn from it.

Really, I'd say these days the best server isn't one where you're stressing over getting popular, but one where you're going as deep as possible trying to really know how the place works. In that case, even if you fail, you will have become more knowledgeable for it.

What makes a server stand out / unique?
I'd argue that again, this doesn't matter a great deal in terms of what becomes popular, but lets see. Identifying what people like to play and adding your own twist, challenge, and gimmicks to it could be fruitful. Find out what a) people what to play and b) what you want to work on, since a server striving for uniqueness can't really get there if it doesn't have the spark of passion behind it.

Connecting to above, it's not just throwing in obscure plugins or even configuring them in a slightly unusual way, it's about forming an experience that deliberately tries to figure out the player's journey and then throw a few wrenches in it so you give players something they can't quite expect elsewhere. You can do this through design, making your server a cohesive experience from the moment players join to how they approach endgame. Know what they do and then change it up. Again, even if it fails, you'll learn from the attempt.

What do you or would you enjoy in a server?
I'm a weird player and I know I'm an outlier, so I'll focus instead on what tends to be played. Survival and factions still take up a good portion, prison and minigames are still big (but if you want to be unique with those, you'll definitely need to put in some thought. Not necessarily money, custom plugins aren't required to make a unique experience). Towny has its loyal following. I've been out of the loop for a while, so that's about all I remember, and I'm not sure what has taken the lead lately.

I like places where I can chat, build very casually, and chat. Yep. That's my gig. And even that I don't do much these days.

any other input you would like to include.
I'll summarize and repeat various bits from above.
- Custom plugins don't have to be made for you to design something unique. In my experience it's not about the plugins you throw in, since everyone does that; rather, how you make them a cohesive, understandable experience that players will naturally get into.
- Community is everything. The best designed servers and the places with the most effort get nowhere without some of that sweet community glue that comes from the right players knocking on the door, usually quite by chance, with slightly better odds if you've made an effort to network.
- You do not need to open with 20-40 staff members with 3/4ths of them being moderators. Moderate for your size. Every timezone or other such metrics is only a requirement to consider if you're dead set on the community thing above and want someone for players to talk to at every given time. It's not because things actually happen to require that many that often.
- Research and know your plugins. 2 years and a lot of things have changed. For example, if you want the most mileage out of permissions, LuckPerms is now the standard, being the best updated and the most feature packed. GroupManager is still updated and functional if you want something simpler, but do not expect it to scale very far or well. Still ,it suits most needs well enough. PEX is also still functional, but its last update predates the pvp update. It's pretty old now. Still works, but it's telling that the PEX creator advises everyone who uses it to move to LP.
- Pairing with above, you should look into and do your own permissions, so you can learn from it and be familiar with everything that is being configured. Something like Essentials is a pain to configure, and yet, going through them one by one might teach you a few things.
- You're asking for pain if you're trying to make a cutting edge server that religiously updates as soon as a version of spigot comes out. It's advisable for your stability and sanity to update when the game version and all key plugins are stable. Unless you're working with something quite small, you're not going to have too many gains from updating off the bat. Players who don't understand that servers don't move as fast as game versions do (when the game versions these days don't even reach stability in 3 hotfix updates) are not very sustainable players, going straight into my opinion.
- I know there was more, I just can't remember. I'm not concerned for length, people without attention spans will have lost me paragraphs ago.

also any cool ideas or plans you would like in a server
Well, I did have two weird ideas at one point that I've since bounced around for years. Never got to making them, and since I'm pretty much retired from servers, I probably never will. Just food for thought, rip from them, take them, ask for more as you see fit, or simply ignore them.
- Nether-based minecraft server. Prebuilt economy, world set up to allow you to earn resources, nasty mobs to face, basically an overworld level of detail experience that is 100% set up in the nether. I don't know why I liked it, but it's stuck.
- Parody staff-hunter's server where the entire gameplay and ranking system is built around people hunting for staff. Yes, this is the weird one. It seemed interesting due to server history (especially the old stigma of this website) and the fact I haven't actually seen a server try to play into the ambition of becoming staff that was so prevalent. I believe it's still going strong, but I haven't checked recently. I know it definitely went downhill and this idea isn't as potent as it once was. But it still could be.