u can go to lvl 60 soloing
only thing u cant do is instances which require u to join up with a group and the best gear comes from instances.
And u also cant do big raids when u are lvl 60 if u dont join up with a guild, and thats the best part in the game high lvl raids
yeah it get better and better as u are lvling up at lvl 40 u get a mount.
but personally I think the most fun is to do the instances which requires a group, so test that out.