I have been running 1.7.5 on my laptop and just upgraded to 1.8 stable. While the normal way to run MongoDB (at least in my work) is to use read-only slaves for analytics, etc., I am still glad to see the single server robustness changes, including optional journaling. I also noticed that in the admin shell, showing databases provides database size estimates.

Another useful change is replica set authentication using identical key files that are placed on each server. You then let one server know about the others (as before). You can read about other improvements here.