As an abstaining domainer let me sum up what I learned:
The shorter the name the better.
Don't use hyphens nor digits.
Dot com is the king for any international project.
In many countries their CCTLD is better than a .com (eg .de in Germany).
Do three tests:
- Radio/phone test. Say the domain name over a phone. There should be no problem spots you should repeat.
- Writing test. Look out for any easy typos: repeating letters and language-specific grammar problems.
- Association test. Ask your friends for their first impression and what occurs to them immediately after they hear the name.
Made up names are ok.
A good rule of thumb is: Make sure there is only one thing the users have to remember.
Eg unusual TLD + a made up name = 2 things to remember. Reduce to just one.
IMO the domain name doesn't really affect success, there are far more important things like founders' skill and passion.
However not observing the above-mentioned rules can make the success harder to reach.