Great sound quality.
A great Bluetooth speaker will deliver room-filling sound across the whole musical range, from treble to bass, without distortion.
Smooth wireless performance.
Your speaker should be able to pair easily with your devices and stream your music smoothly, without interruptions.
Ease of use.
Setup should be straightforward, controls should be clearly marked and intuitive, and the system should allow you to control the music easily from your phone.
Long battery life (for portable speakers).
The best ones can play for 8 hours or more on a single charge.
Signs of quality include a solid metal case (or ruggedized fabric and rubber, for outdoor speakers, waterproof), a stable base and a generally solid feel.
If a portable speaker doesn't offer a sleek, stylish design, it should at least be unobtrusive.