The two most common mistakes are bass and treble

OP:  Always run a long-term home test before buying speakers.
Give yourself plenty of time to climatise yourself to the sound.
You will have years to enjoy your choice not years to regret it.
I am sure you know this.

To all the posts about low ohms and low efficiency, your opinions are nothing but mindless prejudice.  Nothing at all wrong with 4 ohms and 86db.  As has been said, most of the greatest speakers on offer are around those figures.

Forget the specs and judge each speaker on its merits. Use them only as a very broad guide to room matching and amplifier selection. Even then you sometimes get surprising outcomes that shouldn’t work well on paper.

Worst mistake for me is not to hear the speakers in your room with your gear before purchase.
I doubt impedance or sensitivity alone say a lot about the speaker sound. My worst speakers were with 8 ohm and north of 100 dB. Eversince I am around 4 ohm and below 90, increasingly satisfied with every next upgrade. And it is very true they require appropriate amplification. Every thing costs another thing. That’s life.

While I understand some of the arguments around high impedance and high efficiency speakers I also have to ask how often does this happen?  How often does a buyer mismatch his speakers and amps? 

Or is the argument if you aren't buying 16 Ohm/110 dB speakers you made a mistake?

The most important factor is not the necessary well match which is needed for sure between amplifier and speakers, and everyone learns that easily, but the most important factor is the room controls OVER the speaker and the room controls designed AROUND these specific speakers...A fact which is less easily understood and learned...

Acoustic is the KEY..... Not electronic design only and mainly, even if for sure it is important....In all audio thread all is about electronic design market....BUT it is secondary to acoustic... Nothing exceed the power of acoustic to give the most rewarding experience with ANYTHING well designed with an optimal S.Q./cost ratio....

And acoustic cost TIME not necessarily money.....And remember that acoustic is NOT only passive treatment but more than that, activation of the room...

Saying anything opposite will induce complete misunderstanding in beginners and worst will ignite upgrading chase of our own tail or worst, chasing of the moon....

Underestimating acoustic is the common error in most audio threads....I already paid the price for ignoring that myself....Nobody advised me CLEARLY and LOUDLY about this fundamental task....Then i speak to beginners here....Beware...

Dont think about a 10,000 bucks dac, speakers or amplifiers to be SOLUTION... Think about the time to create acoustical settings for your system.... It will cost time not so much money....

The 2 main factor about speakers/room are ASW and LEV not bass and highs....