Picard's boasts to Q result in the Enterprise being thrust into a conflict with a remorseless and extremely powerful adversary.
To my mind, this is the Borg used to best effect - no "Queens", "Spheres", "fallen members", movie deals, etc. Just a totally different kind of opponent.
As soon as the Borg were over-used in every series of the franchise, they lost all their impact. Here, it works, totally alien, totally un-interested in anything close to human motivation, completely mindless in the ability to scan and assimilate anything they come across. Lots of drama is built up in the plotting and execution of this episiode.
And that's how this installment is written, with nice references to the Borg's past terror and visual examples of their invulnerabilty to the defenses of any opponents. Of course the temptation to use them over and over again would be great, but at least in this first installment, Q introduces the Federation to something that is seemingly horrific but at the same time is diabolical in that it DOES make sense in an impersonal way.
A very clever idea in science fiction for television.