Newbie here so I'll take the plunge.
For me I've taken a very conservative approach, and pay all cash for my deals. I currently have two properties. Given this I'd have to say cash is what is holding me back, but with a big caveat.
I still have my FT job, a stay at home wife and two children to consider whenever I make financial decisions. Given that I maintain a 6 month cash reserve for personal financial emergencies that I won't touch for real estate investments. I also continue to fund my 401(k) at the maximum amount and also both of my kids college 529 plans.
I have a plan to purchase 2 homes a year for the next 5 years then do an evaluation to either maintain at 10 homes, grow larger or wind the business down.
Now obviously if I had a larger pool of cash I was willing to part with that plan might look much different, but given my risk tolerance and cash availability this is what I'm comfortable with at the moment.