I would honestly say talent.
Any band can have a singer, a number of people playing instruments, the ability to string words together to form lyrics, and the capacity to put it all together, but what really is most important is that they're able to use all these effectively.
Just for an example, let's take a look at Pearl Jam, one of my favorite bands. We have Eddie Vedder, somehow who is able to compose mystifying lyrics and wow the audience with his guttural, throaty voice. Jeff Ament, Matt Cameron, Stone Gossard, and Mike McCready, all exceptionally talented musicians, have the remarkable abilities to really make the most of their instruments and almost "speak" through them.
Now, take like (just off the top of my head) Plain White T's, a fairly young group who (in my opinion) isn't worth the attention they receive. Their lyrics are simplistic, their singer seems to exude little to no emotion during their songs, the melodies and hooks that they provide with their instruments lack any real thought or layering and seem to consist of providing as much sound as somebody pretending to play those instruments could also do.
Now, this is all just my opinion, so take what I say with a grain of salt. And to any Plain White T's fans that I may have offended, I deeply apologize for unsettling anyone who thought the group quite good :tongue: