Obviously a good description is part of making any character unique, here are my thoughts on what makes a good description.
What I think a description should contain: Body shape, skin tone, face shape and features.
You should also probably include if your character has them: Smell, makeup, facial hair.
You can include more obviously, but those're the things that everyone has, that aren't covered at all by the coded description stuff.
Things you shouldn't have:
This is going to be slightly more controversial, but as I see it a description should apply all the time, if you're drinking in the bronze or doing the naked polka dance. So it shouldn't include things that are only applicable in certain circumstances. Like for instance. "He always seems happy."
Mannerisms are also something I'm not overly fond of, like "He smiles a lot." Because really, you can just actually make your char smile a lot. Mannerisms are something that can be RPed, descing them is just kinda lazy.
Info people can't know: Stuff like "He is a dangerous man." Or "He looks like he's looking for adventure." I don't know these things, and if you show them to me, the question is, how do I know? If you keep a description purely to things everyone knows by looking at you then it's uniform, anything else you can RP. Like "say Hey, I'm really looking for some adventure!"
Skipping the why: Kinda from before, saying someone looks scary, or cool or anything like that is sorta oblique. Different people will find different things scary, or cool, or attractive or whatever stick to just how someone looks, and let people draw their own conclusions. I remember seeing one char who had something about their eyes showing an innocence lost and a sound resolution to do good. I have never in my life looked at a person and gone wow, they have eyes that kinda look like the eyes of someone with an innocence lost and a sound resolution to do good. Which is just to re-iterate, stick to what people can actually know.
General tips. Write well, ok so this is hard for some, but look at other peoples descriptions and you should start getting and idea, try to avoid repeating yourself, and reusing the same words. Also try to keep it flowing, rather than a shopping list of features, .f your descs or make sure the lines are short enough for everyone.
Some people may disagree but I think it's best to keep descriptions as short as possible, the smaller it is the more likely people are to read it, and the more the things that are put in it will stand out and be remembered. Personally I usually aim for like 4 lines or so, something I almost always never actually attain. You can keep size down by not repeating yourself too much not including stuff that's in your coded desc stuff already hair, eyes etc, and not talking about stuff on your character that's perfectly normal, if you don't mention it, people will assume it's normal anyway.
Tips for avoiding overly long descriptions:
There are some people who feel that the length of a description is a determinate in how good it is, following from the logic that a description is better than no description, the more description the better. They're wrong.
The point of a description is to convey to other people what your character looks like, if it's so long and cumbersome nobody reads it, it may as well not exist, in fact it's worse than not existing cause it's likely to spam other info about you off the screen. Ideally a description should be able to convey what someone looks like in a few seconds. For instance when I meet someone ICly I usually start with a generic greeting, then look at them so that by the time my turn for say/emote comes back around I have a fair idea of how my character is going to react to them. Most people on meeting someone on fire, or unusually tall, or naked, or wearing some particularly noticeable piece of clothing are going to comment on it, it's certainly going to affect their opinions of the person and thus how they react to them.
(BTW: A lot of people don't do the looking thing, and it is another thing people really should try to do more, RPing with a naked guy for 30 minutes without noticing they're naked is pretty hard to explain away ICly. Also there's a somewhat silly trend in people deciding everyone looks how they want to look, oh that girl want to sleep with me? She's crazy hot! Oh that girl wants to kill me? Boy what an uggo which is largely aided by never actually looking or paying attention to the look info of other characters)
So we can see that it's important you convey information quickly and easily, preferably with the most important information first in case the person only has time to skim. Problems with cumbersome descs come in three basic flavours:
It's too long, that is there's just too many words. There are some people's descs I've seen who even though I've been sitting bored in the hospital and knowing their desc is in my backlog I still can't bring myself to sit through reading it.
It's too tall, usually goes along with being too long but also often because of excessive use of paragraphing or focuses makes the start info on someone's look screen scroll off the screen, either straight away or after an emote or two.
It's too hard to read. Bad writing can make a desc a real struggle to get through, though if it's short people are more likely to read it even if it sucks.
Things to avoid:
Redundancies: You have a hair desc, you have an eye desc, as well as height and age, generally speaking there is really no need to repeat yourself. Flowery language: Not nearly as cool as you might think it is, tends to slow things down and it's really not likely your desc is going to be entered into the Pulitzer prizes for poetry anyway.
Judgment statements: Anything like, good, nice, pretty, attractive, beautiful etc is almost always a mistake to use because they don't really tell you anything. Almost every man might agree that both a tall, Caucasian, curvy girl and a short, Asian, slender girl are beautiful, but saying they're beautiful tells us nothing about why they are beautiful or what they look like.
Assumed information: If something can be assumed from either a normal person or the rest of your desc there's really no need for it. If I see a character who hasn't described their teeth I assume they have normal, fairly straight and white teeth. If something is implied from the rest of your desc it's also likely not needed. E.g. if you have black hair people will naturally assume your facial hair is black also. If you are a demon covered with say wooden skin people will assume it's wooden everywhere unless otherwise specified.
Occasionally someone will try to interpret your desc or rp in a way that better suits them, e.g. deciding because your char hasn't described their teeth they can decide your teeth are yellow and ugly. But they're fucktards and horrible RPers and should be ignored except when throwing cabbages are at hand.
Focused Descs: Focuses have their place, and can be useful, but they should be avoided if possible. A focus makes a desc taller, usually longer, almost always less pleasant to read, and impossible to decide in what order information is given. You should avoid using a focused if:
The area in question is something that you can't foresee yourself covering. For most people this would be face, hair, hands, neck. If something changes with your character to make them start covering that area sometimes you can always move the information to a focus at that point, but it's pretty unlikely to occur.
It's redundant. Generally that's going to be hair or eyes, eyes there might sometimes be a cause for a focused if your eyes are hurt or red or have bags under them etc and you might or might not be wearing sunglasses to cover that up. With hair though, you have a hairstyle command that allows you to easily change your hair string with almost no length limit so there's really no excuse.
It's repetitive. Similar to redundant but even if there's some new info there, try not to repeat other info that's been said already. Also if the area is somewhere that's almost always covered you might not need a focus either, unless it's very important. People who put on 10 different focus descs for scars etc are as annoying as hell to look at if you have x-ray vision or they're naked. It's easy enough to emote something like emote peels off his t-shirt revealing the thin whip scars criss-crossing his back. Which is generally better and allows you to save focused for stuff that's more important. That way people are more likely to read the focuses you have.
You can also group together focus descs on areas that are going to be seen together, for instance on most guys if you can see their stomach you can also see their upper chest and back. So instead of doing three focused, just do one. It'll be easier to read and less spammy. Of course that doesn't apply if you're a guy that wear's cropped tops or jackets with no shirt underneath but in that case you have more serious problems than focus descs anyway. Personally I tend to aim for a desc that's about 3 or 4 lines long, and I never make it. It's fine to go over just try to get the thing as short and pleasant to read as you can. If something's important to your character by all means put it in even if it increases the length, just be sure to put some effort into keeping that length down if at all possible.