I have a reasonable amount in common with her, in terms of stuff we've experienced in our lives, where we grew up, and the fact that we both value security fairly highly.
IMO having a lot in common with a partner isn't essential, it's more important that you are tolerant and understanding of your differences. She's not really a football fan but can sit and watch a match with me, she's not into gaming really but understands that I'll stay up to 1am playing games. Conversely I can live with going to the odd chick flick at the pictures every now and then, and don't mind her obsession with fluffy toys and the like. We're both quite forgiving in general and never have serious fallings out.
There's sometimes I wish we were more alike, of course. I wish she had a less obvious/tame sense of humour - there's other women who make me laugh more.
...and then there's sometimes I wish she was a bit different. We're both relatively introverted which means that we're probably viewed as a fairly boring couple by others. She has very little drive and doesn't push me career wise, she's quite happy to just maintain the status quo and live the same old life eating the same food and watching the same tv shows. I know if I was with someone else I'd get more 'life experiences', be out there doing new things and meeting new people, but I've always tended to value love and support over that type of stuff.
What someone mentioned above about it being important to not feel uncomfortable during periods of silence, that very much rings a bell with me. We talk a hell of a lot of meaningless drivel (just really boring everyday stuff like planning meals for the week, what we've done during the day bla bla) but it's pretty rare that I feel like I need to say something to keep her interested. That certainly isn't true for all women.
I guess at the end of the day it all boils down to what one wants out of a relationship. I basically turned down a 'party girl' who most would brand a better catch than my partner, because I knew we had different agendas in life, and that as fun as she was, I couldn't see a long term future with her. There needs to something more than just a physical attraction and a shared sense of humour.