The next one we might outsource is book keeping.

In your example the S&P investor invests zero time (save for a few seconds of mouse clicks), whereas the RE investor must invest significant time managing his properties (or managing the PM), facilitating financing, bookkeeping/accounting, etc.  

But there are a number of tasks that fall outside of systems, like bookkeeping, paying non-reoccurring bills, handling maintenance calls, etc.

I've known lots of investors who owned lots of stuff, but weren't able to dig their nails in and hang on and lost it all.So, maybe a good retort to these inquisitive minds might be "The number of properties one owns isn't as important as the cash flow one receives, but talking about one's income isn't polite so I only discuss it with my wife/husband/cpa/bookkeeper/cat/dog."

Communication problems, bookkeeping problems, tenant issues and the dreaded repair cost issues and misunderstandings.  

If tenant is credit investment grade BBB- or better than Standard and Poor's is analyzing them constantly for debt load obligations versus liquidity on the balance sheet to see how healthy they are.Also even if not in lease sometimes with stock reporting you can get the financials you need.Tenants do not like reporting too frequently as it can increase their bookkeeping hours expenses.If tenant is not investment grade I want everything I can get on them to constantly monitor the health of the business.

Does anyone use Tenant Cloud for their actual bookkeeping as well?  

Make sure you have the following:Greatinvestor-friendly agentsMultiple contractorsto run bids throughMultiple lenders andhard money lenders to quote youA great real-estatespecialized CPA (and bookkeeper)

I bet that system for tenant prepayments will apply in other bookkeeping situations as well 

And a Certified Bookkeeper.