For me the biggest issues are the fact that as I get older I the narrower my circle of friends get. Most of my friends are girls, single girls, and this is GREAT. But as the pool of single men is shallower than of women and married men don't make great friends for single girls it's easy to get stuck in a bit of a rut.
Oh that and the fact that the older I get the less likely child bearing looks. I'm by no means off the clock yet but I have to have more faith every year that things can change.
I guess for me, it's figuring out where I fit in! With so many of my friends in serious relationships/married, I often feel like a third wheel. Now I'm starting to see the first wave of my friends getting divorced! In that way I feel quite lucky to still be looking for the right person.
The biggest issue for me right now is feeling like if I get too involved in a career, I may miss out on finding "that guy." I want the typical package of a husband, kids, my own home... but not yet! I always worry that if I wait to late I might miss the boat.