When I began releasing my levels 5 years ago, I didn't care about good design. I was happy that my levels looked quite pleasing to the eye, there were no tile bugs, every platform was accessible in one or several ways, and there were many pickups spread randomly around the entire map. I didn't play multiplayer either so I couldn't know what exactly contributes to a good level.
After that initial period, ambition to become a valuable level designer made me try to understand why people consider one level better than another. I studied top rated levels, played online, listened to critique. I slowly began to grasp how everything affects level quality. I started following every useful piece of advice I could find and finally my J2O downloads gained appreciation and gathered high ratings. I learned from the best level creators and blindly accepted existing standards.
However, these days things became yet different. Doubts started appearing in my mind and I begin to question. Everybody on J2O is stuck with the same idea of what makes a level good. Opinions vary a bit regarding details; is a bit of camping really an issue?; how good exactly should flow be?; are the majority of Clan Ladder map pool levels complete bull crap? Regardless, I see tons of issues that are never addressed and, in fact, appear to be encouraged, and the best of level creators still implement them in their levels. Today I decided to start this topic and get a bit of a discussion running, and hopefully encourage JCS users to change their habits. Below I'll list and briefly explain the things that irk me the most.

"This level is huge, so I'll give it as many as 5 power-ups."
That's the mentality that killed most JDC events I attended. These days events may have anywhere between 16 and 32 people and guess what: 3 to 6 power-ups will never suffice for that many. How many power-ups would you place in a 4-player level? 2? 3? That's at most 2 players per power-up. What makes you think that 5 players per power-up will work well in a larger game where, additionally, people die a whole lot more due to the deficit of carrots? Where does the thought of adding 5 slowly spawning carrots in a level intended for 15 people come from?
What I want to do in battle mode is grab a power-up or two, use weapons strategically depending on situation, then run away and seek a carrot when on low health. How come all levels I've seen during events instead encourage creating a clusterfuck of 6 people around a single power-up spawn point that finally, after 20 seconds, generates the monitor, only to be taken by one person on low health and lost within the next second? Carrots become completely obsolete because you're unlikely to gather anything valuable enough to care whether you'll die, and even then, there can be so few of them and spawning so rarely that they're not worth the wait.
I say: make the power-ups and carrots rain. Place 15 or 20 of them, it doesn't matter. The more there are, the less campy your map becomes, the less luck is involved, the wider the strategical possibilities become. Don't stick to your traditional long generator spawn times. Pickups with a short spawn time are more likely to be found by people who are new to the level and, unlike you may think, won't result in increased camping.

The key item is secret.
That's just ridiculous. I really thought people would get over it by now but no, we're stuck with the awful idea started by Battle 1 and Diamondus Warzone. Secrets are fun. It's fun finding secrets and it's fun using secrets against your opponents who don't know them. But for the love of God it's not fun fighting against a person who knows how to obtain a seeker power-up, a shield, or a full energy carrot when you don't. I've seen many duels where one of the players didn't know where an important pickup was and they were just painful to watch. The same happens in large games, to a lesser extent, but it still does - I notice that many players who know important secrets score way above their skill level, and I would know best because I tend to be one of those players. When you create a secret, make it a secret path, or a stack of ammo, or even one of many power-ups, but not the most important item in the level.

Overpriced coin warps
They're not uncommon. In fact they're almost every coin warp I've ever seen. 50 coins to collect in a level that has 40 coin generators in total, designed with about 8 people in mind? That just renders them useless. Coins are not even a top priority in a battle. They don't result in immediate gain. You should be able to run around the level and eventually notice "hey, I have enough to enter the warp", then get a cool pickup. If no more than one person per match manages to enter the warp, how worth it can it really be? If you run around in your level alone and need a minute to collect enough coins to put them to use, think about how many players your level can fit, then multiply that number by the minute it just took you. That's roughly how long one will have to stay alive to enter the warp. Do you really think that's going to be possible and worth the reward you offer? I suggest placing way more coins in levels that require them, and optionally reducing the awards. A coin warp really doesn't have to take you to the most powerful item in the level.

Lack of landmarks
Many designers create really beautiful huge levels. You've just got to love running around those and seeing all this eyecandy that must've taken ages to create. The thing is, it tends to be consistent throughout the entire level, not to say identical. When you're the person who created the map, you know exactly where you are when you see an area, but you can't expect the same from players if you make everything look the same. Instead, make various places in the level differ. Introduce major changes in eyecandy style depending on location, make terrain vary, change layout style, add a structure so characteristic that it can't be missed. Simplifying navigation is important and makes a level more fun to play.

Food pickups
Let's face it, sugar rush is nearly useless in most game modes. If you're going to allow it by adding food in your level, and you don't have a gimmick that utilizes it in order to obtain some further pickup, at least place enough to make it common. Spread those apples and cherries all over the level so that even when 16 people play it, one can still easily rush after a minute of running around. Maybe that will at least allow for two or three sugar rush kills per game.

That's all I could think of for the time being. It may not look like a lot and over half of it says "add more pickups to your goddamn levels" but I'd still like to see what you have to say about it. Am I wrong? Is there anything else that people keep doing to their levels while they clearly shouldn't and nobody ever brings it up as an issue? I'd like to see some discussion so I can adjust my ways of level design accordingly, and I wish this thread convinces some people to try stepping out of their comfort zone and creating something different.