The difference is... ED stands for Extra-Dispersion. It has to do with the coatings, and element groupings, as well as the amount of elements in the lens. The ED is just a much better lens. Most of the time the ED's will be made of heavy duty metal. The D's and and G's are made mostly of plastic.
Cheaper made, cheaper bought.
Kind of. It actually stands for Extra-low Dispersion glass.
What this means is that standard fluorite optical glass has several inherent weaknesses.
One: It is easier to scratch/chip/break.
Two: It does not like large aspherical shapes.
ED glas on the other hand is as close to completely bubble free as any glass ever made and is extremely hard which allows it to be used in front elements of lens...although most ED lenses do have a flat miniscus optical glass built in on the front as sort of a permanent protective cover.
The BIG difference however is that ED glass has a different optical index due to the extreme concave shape it can take on,,,this allows all the colors in the visual range to focus at EXACTLY the same point...that being the plane of the CCD or film. The benefit is that there will be absolutely no fringing or ghosting of colors in the image.
This can be achieved with regular fluorite glass but if you make a lens with a large enough aperture to be considered even moderately fast the compromise with fluorite must be made to have an extremely fragile high quality piece of optical glass or a durable lower quality piece.
Now is it worth it? That depends. If you are using a lens with an optical length of under 200MM no. At 200MM it is about a push...IOW a 200MM F4 or slower lens will be fine with a fluorite glass lens. A 200MM F2.8 OTOH really needs ED glass and the difference between an ED 2.8 and a fluorite F4 is quite noticeable.
At optical lengths of 300MM or longer you are IMHO kidding yourself if you use a non ED lens and want high quality results.
Yes ED glass gets pricey, especially in something like a 300MM F2.8. An 80-200MM F2.8 ED zoom or a 300MM F4 ED is high but within the reach of a serious hobbyist. Keep one thing in mind...you are buying a piece of equipment which is virtually indestructible and actually has resale value unlike most hobby items. As an example if you had bought a Nikon 80-200MM F2.8 ED lens 15 years ago for $750, today it would still sell for around $600. If you would have bought a Nikon 300MM F2.8 ED 25 years ago for $2,000, today it would still sell for about $1,500.
Today a 300MM ED F4 and an 80-200MM F2.8 ED will both sell new for about $900 each, a 300MM F2.8 for about $2,800.
Unlike pool where a $2,500 cue won't make you a better player than a $200 cue, there are shots that you can take with a fast ED glass lens that are just impossible with slower glass...no matter what skill level you possess.
LWW