If you are a classical player and are looking for a high-quality opening, the Classical Variation of the Slav Defense could be something for you. It is a solid, but by no means boring weapon against 1.d4. In many lines, there are enough imbalances upon which Black can build winning ideas.
In my opinion, the Slav and the Queen’s Gambit Declined are more or less equivalent openings, with their specific pros and cons. I see the Slav benefiting from the fact, that it avoids the Catalan or the Exchange Variation of the Queen’s Gambit Declined. And against the English or Reti-type setups, live appears to be somewhat easier.