Time. I just started on this so my biggest barrier is the 15 or so years it will take.
Time isn't a barrier; you choose when you FIRE. Choose 4-5 years instead?
;)
I could FIRE tomorrow if you send me $1.25M. 4-5 years is just not possible. 15 years is achievable. The way I think of it, I spent 20 years in school and will spend 15 years in a job so that I may enjoy the last half of my life my way.
Between SO and I, we have 30k in net worth. Our income is about 70k/yr combined. We spend somewhere about 30k/yr, so figure we can save about 20-25k/yr it's going to take a while to get to FI. 5 years from now, that money (in today's $) would be something like $150k. A great start, yes, but not enough for more than an ERE lifestyle. I like my fancypants (and totally reasonable) ~30k/yr lifestyle thank-you very much! Actually, if I didn't work, I'd spend more than I do now on account of insurance unless I can get HDHP under 250/month in which case it's the same.