I think the first and most important thing to remember about writing descriptions is that this is the part of your listings that is taking the place of the customer being able to pick up your item and examine it, AND consider why it is something they want to buy. Therefore, approach your description as if you were telling them all of the different things that they would observe had they the opportunity to do that and point out what is so great about YOUR item.
Writing descriptions needs really a two-fold approach. First, you have to be the creator of the product and describe in detail what it looks, feels, smells like. You also have to be the customer though and think in terms of what makes YOU buy something and translate that into your listings. That's the tricky part so I'll start with the layout and details of items and then tackle how to write to/as a customer.
Finding the balance of "just the right amount" of information might take a bit of trial and error. Basically, you need to include all pertinent information, but not overwhelm your customers. If you need to include a lot of information, make sure you are organized and logical. The very first thing your descriptions need is a brief one or two sentence introduction. This is very important because this intro has weight in searches both through Etsy and search engines AND it is what will capture and encourage a customer to keep reading. To optimize this, you need to include all your most important keywords in sentence form and make sure a summary "what is it" of your item is made. You will expand on it later... this is just to "punch it out there" so to speak.
Here is an example from my shop:
"Snap Belts keep your lean toddler or preschooler's pants up in style! Simple to use... just snap the elastic through the back belt loops for a snug waistband"
This tells people right away whether or not this item is what they might be looking for. Do they have a preschooler that needs a belt? Easy to use. Elastic and snaps. Answers the question "what is it" briefly and includes vital keywords (which are mirrored in titles and tags... as a side note, doing this increases the weight your item will have in Etsy searches!)
When you write your first few lines, look under the "description" box while in edit mode and take note of what appears in the google preview. Does that include all your keywords and could stand alone to catch a customer's attention?
The next thing that should come in your description is a quick rundown of the item specifics in order of what would be most important to your customers. Don't be afraid to use bulleted lists and spacing to make sure things are organized and succinct.
End your description with a quick conclusion. This might include whether or not you do customs, how to contact you (perhaps a link to the conversations feature), or other information that isn't vital to the specific listing but you feel is important to share.
DETAILS (the middle of your descriptions - in NO particular order... you should rearrange these details according to the most important info for your item and combine them of course when possible to keep things as concise as possible... this list is just to get you thinking of things you might have missed or could add)
You can't rely on your photos to describe your item for you. The two need to go hand in hand. You description should still include things that might seem obvious to you from the photos.
Size. Include actual measurements if possible, and certainly general size if it is a clothing item. You may also want to include weight if, for example, it is a heavy pair of earrings or with have to ship at an "overweight' rate.
Color. Remember that computer screens may show color differently to different people (even our desktop and laptop show colors slightly different). Use adjectives whenever possible. If you aren't familiar with how a color might generally be described, there are so great color charts on wikipedia when you search more general colors, or go through your kid's crayon box and see if you can find something similar! For example... is your item Kelly green, Sage green, Emerald green, Grass green, Mint, Pistachio, Forest green...
Type. Is there a "proper" word for the item? If it is a girl's outfit for example, is it a pillowcase dress, peasant dress, tunic, romper, bubble romper, bodysuit (do not use "onesie" as a decription or tag unless you it is Gerber brand, this term is protected by trademark), kimono style, wraparound, etc.
Material. What is it made of specifically? Silver plated, silver, pewter, stainless steel, nickel, etc. Acrylic, oil, latex, watercolor, etc. For my nursing necklaces, instead of just wood I specify hardwood, instead of just oil I state that I use Organic Sunflower oil, instead of cord I say black nylon, and in each one, the stone bead is described by type of stone and color.
How is it made? Serged, french hemmed, woven, quilted, hand-tied, etc. If there is a process to how it is done that makes it unique, include that so that your customers know what you've put into it to make it so wonderful.
Care instructions. Is your item machine or hand washable? Include specific details such as "wash on cold, lay flat to dry". Will it need or might it be desirable to treat it later on (can it be polished or rubbed with oil to maintain the beauty?)
I'm sure there are lots more things that can be included in the details of specific items... if there are any you want to share please do so!
NOW THE TRICKY PART
How to use your descriptions to hold-on to a customer and convince them that YOUR item is the one to get! A professional description and photos says a lot about yourself as a designer/creator and about your items. I think most, of not all of us, have competition here on Etsy that makes similar items so how do we encourage people to buy our product?
Well, first I want you to forget about competitive pricing (and if you haven't already read it, please read the post from several weeks ago on pricing to find out why!)... and I want you to think about what makes you buy one thing over another. We buy Campbells soup. Why? It's thicker, has better flavor, I like the recipes on the back of the can :) , and I like actual chunks of veggies in my soup rather than tiny shreds you get in some kinds. What makes you buy one type/brand of diaper over another? What makes you like a specific pair of jeans? What would sell you on a set of sheets or new towel? What do you like about your car? (I liked how in our old car there was a cover over the mirror behind the sun visor... our current car doesn't have that and it's something I'll be looking for in our next car! Sometimes, it's the little things!) :) We are all customers, so it's just a matter of shifting gears from be maker, to thinking about what appeals to customers.
You should also talk about the craftsmanship that make your item special and stand out from others. Are your wooden toys cut, sanded, and sealed by your own hand? Did you dye your own wool for those knitted overalls? Do you use organic products to make or treat your products? Are the edges of your ribbon heat-sealed or stitched to prevent fraying? Are your materials fair-trade or local? Do you include special packaging for your items? Is the material you use designer? Think about what made you choose certain materials or processes and tell your customers about it! They won't otherwise know what goes into your product unless you tell them.
Describe how your item can make their life easier or better. Is your design more efficient than others for some reason? Can it save them money or is it better for the environment and how? Does the materials you use offer a stronger or more long-lasting product? Why did YOU design it the way your did??
There is one more thing I want to mention... while using the copy feature is fantastic for saving time, make sure when you do so, that you read over your description again and just make sure that all the information you included is still accurate. You may accidentally copy a mistake and taking a quick minute to review when you copy or renew an item will ensure that you don't accidentally make yourself look bad by reposting something that isn't accurate. :)
I hope that this has given you something else to think about in terms of writing descriptions that you might not have thought of. For some of us, we ramble too much or our descriptions could use some more organization... for others, more details are needed to really express completely what we have to offer.