Seems like bass players need to have that extra special quality in their playing to really stand out in their respective bands (i.e. Geddy Lee, Tony Levin, Steve Harris, etc.).
All instruments are equally hard to master because there is an endless amount of knowledge and skill for each one of them.
I will say this, however: Guitarists are the most self-absorbed. I find they usually only focus on guitar in music, and think of themselves as a higher power in bands. I'm, of course, speaking on a general level.