In my book 'smooth' is two things- a lack of high frequency emphasis (however we are not talking about anything being rolled off), and a lack of odd-ordered harmonics, which is the source of 'jagged' sound that others here have referred to.
Odd-ordered harmonic content is often described as 'hard', 'brittle', 'harsh', 'clinical', sometimes 'overly detailed' (which is impossible if there is no brightness) and now 'jagged'.
You can have smooth and a lack of detail, but in my book when a change in the system causes smoothness and **increased** detail, then you are on to something. The smoother, the more detailed without losing speed, the closer you will be to the recording itself.