This problem with confluence startup lists 10 separate and different reasons for the exception. This is laziness on the part of someone who likes to code in Java and really quite inexcusable for any installer.
As a System Administrator who is installing for the very first time, encountering this, finding "you installed as root" (well of course I did, there's no instruction not to and I wanted the global service arrangement set up) as one of the PROBLEMS leads to a great deal of annoying bug hunting, reinstallations, and mucking about. That entry wants to be made clearer.
Actual problem I had was with the DB installation. A screen where it says overwrite DB and it stays "live" on the page. As in you navigate away from the page in the nigh-on 5 minutes it takes to overwrite the values and it breaks the DB.... and you get the yet another error and cannot get a running system.
Only way to clear it is to wipe the /var/atlassian directory clean and restart the configuration.
This is, in terms of normal expectations of an installation process, just a wee bit broken.
Atlassian does not currently permit comments on the ten reasons page so I can't actually tell them what is wrong there. Nor have I any spare time to spend on explaining further as I had to spend it figuring out what SHOULD have been explained better elsewhere.
Now I know better, but I have (here) done my best to alert Atlassian that they need to revisit their 10 reasons page and their DB overwrite process. Getting some programmers to provide unique exceptions would not be a bad idea either, but the verbiage is cheap to update and the DB overwrite process only wants a "do not navigate away" notice that is kept alive by the process. Not big changes, so cheap and possible.
If comments on the ten reasons page ought to be working then that's another thing to be worried about. :-)