For myself, I have recently discovered two important aspects (my 2 cents) that I feel are most important after you have learned the basics:
1. Sente
2. Reading
Regarding Sente, everybody knows about it and knows that it is good. The point I am trying to make is that the importance of Sente is easily under-estimated. It is so easy to get distracted in a local situation and miss the right time to take sente. Sente is what keeps you in the drivers seat. It keeps your opponent under pressure and gives him the opportunity to fall behind. So you have to ask yourself 'Can I take sente?' literally EVERY MOVE IN THE GAME.
Regarding Reading, it is similar: everybody knows that reading is important. The point I am trying to make is again: Reading is important almost EVERY MOVE IN THE GAME. (I say almost because there are exceptions where you do not have to read because there is only one move that can and must be played from a tactical or direction-of-play point of view. Or you are in the middle of a joseki. Or you cannot read because the situation is too complicated and you have to rely on your gut.)
It is so easy to skip reading and play a move
- that you think works (but doesn't)
- that kind of works but is sub-optimal and there is a similar move that works better
Reading is what makes your moves powerful! If you do not read, your moves are weak!
So you have to get into the habbit of enjoying to read.
How to do that? -> Reading has to become effortless.
How does something that is hard become effortless? -> By training and practice.
Some people advise to do lots of tsumego. For me, that never really worked because I quickly lost interest. I do not enjoy doing tsumego, but I do enjoy playing games. Instead of doing lots of tsumego, play lots of games - but: do some reading on every move. And I really mean EVERY MOVE (with the above mentioned exceptions).
If you are too lazy to read, don't play the game and wait a little longer until you can get behind the 'read-on-every-move' part.
Hope that helps.
