How do you know what a good speaker is?

@kenjit - one more important speaker attribute - i.e. LOOKS !

"Generally" - this has little to do with the sound produced, but if a design pleases the eye of the listener then it can play a significant role in a listener’s final choice.
e.g. Speakers with those bright cast aluminum chassis or those spectacular Horn speakers, tend to look more expensive and will probably win out over a more mundane design regardless of performance

But back to the questions in your original post...how can one judge speaker performance?

It comes down to very subtle differences in what the listener is able to discern. Basically, how well suited is that specific speaker suited to the listeners ears.

Everybody’s ears are very different and therefore, we each hear different subtleties from one speaker to another.

The tools for measuring speaker performance are not capable of "rating" the tonal qualities of a speaker AND take into account the acuteness of each individual’s hearing. It would be nice if one day, someone invents a Timbre Meter :-)

Technological Advances in driver design can give a brand a perceived "edge" - i.e. some listeners can discern differences while others just like the idea, or must have the latest in technology.

e.g. the Plasma Tweeter Driver - supposed to provide excellent reproduction in the very top end, but my ears could not discern it’s advantages over a more conventional design, so for me it was not worth that additional expense - but they did sound very good :-)

Unfortunately most of the results from today’s wonderful developments in speaker design are lost or coloured in the inadequate listening environments of our local audio stores and homes and the systems & cables they are connected too.

e.g. - my speakers were designed and tested with the aid of an anechoic chamber at an independent institute using the most sensitive tools to "gauge" performance. Results from SPL meters, oscilloscopes etc.. show the speaker replicate the input signal to a high level of accuracy.

But that was far from the listening environment at the store where I bought them and totally different from my house/room where I listen. And then MY ears provided that final judgement.

How do I asses speakers? - personally, I have been exposed to the playing of "live" instruments for most of my years and have become very familiar with those sounds. That is what I use as my reference.

If a speaker can reproduce those sounds to a degree of accuracy as I recall them and to what I deem as an acceptable degree of accuracy, then it will probably be the one I select

I no longer look at spec’s, because as you stated …
- they are ALL so very similar.

I Trust my ears - they are the best tool each individual has and they can save you a ton of cash. Why pay for something you cannot hear?

FYI:
- I always take my amp and cables to audition speakers
- Trusting my ears has lead me to buy only two pairs of speakers in nearly 35 years for my audio system.
- My speakers are mundane to look at - but they sound great

Unfortunately - there is no real answer here, but maybe some insights?

Happy Listening - Steve

A good speaker is one that puts a smile on your face when you are listening and makes you want to listen for hours. 

Tom

The polar response is indeed important, but the off-axis measurements done by Stereophile, SoundStage/NRC, etc. use is more telling. 
 
Good vertical response is important too, say withhin a +/-10° window; this is where speakers like the Tekton Double Impacts do horribly in, you need to be on the reference axis or else the response changes greatly. 
 
Dynamic compression is also important, how the speaker measures at low and high volumes. 
 
How its spectral decay and transient response is also are important. You want fast transient and an even decay that’s also decently fast. 
 

The reason why the majority of speakers offered today have good frequency, dynamic, timbre and spacial responses is due to competition from a wide variety of designers and companies, each with their own version of the answer to your main question. Each individual carries her/his own set of subjective, objective, emotional, quantitative and qualitiative values, opinions, ears and eyes as to what forms the so-called "ideal." Even identical twins have their own sets of preferences simply because it's physically impossible for any two people to always occupy the same place and time, and experience the exact same things, simultaneously.

I celebrate that we get to live in a time when there is an abundance of choice, that there is no "ideal" or absolute, and that part of the pleasure is in the hunt to find the speaker, and, as importantly, its associated components and listening environment, to form the system, and, as Tom says, puts the smile on my face and makes me want to listen for hours.

I’m convinced that a “good” speaker is only good within a given “system”. Same speaker moved to a different room and paired with a different system will not be as “good”.