I think it's important for the ambience of the music to match what is happening in the game. To me, the music is very important in immersion. Having a weak ost is a big hit to a game for me, while a good ost can make a decent game feel incredible. It's rare for a modern game to straight have 'bad' music that you have to turn off, but a lot of it is generic and forgettable, and was clearly an afterthought in the production of the game.

For example, I'm currently playing Albion. None of the music is bad at all - it all has a high production quality and sounds fine, it's just outside of the login theme, it's all incredibly generic and I usually just have the OST on mute.

It does take away from the game, when it all revolves around high risk pvp and dungeon diving, yet all the music doesn't convey any sense whatsoever of danger. The music makes the game feel more like a chill pve farming game, rather than one where you have to be alert at all times.

Most of the best ost's I can think of include multiple styles of music with a wide range of instruments, emotions, tempos (ffxi, ffxiv, ro are examples I have in mind), and aren't afraid to be bold with their music and in your face with the melodies and energy, even with calmer areas like fields and towns.

I am also a sucker for leitmotifs that represent characters or areas, and having these melodies appear elsewhere. During boss battles, PSO2 would have a high energy remix of the main theme from PSO1 slowly fade into the boss music right as you were just about to win, which was such a good touch by the composer, and made those fights iconic for me.