I got errors upgrading to 2.0.1 some 1.9.10 sites.
These sites have been originally installed as 1.6, years ago, and later upgraded to 1.8 (I skipped 1.7) and then to 1.9
I see that mdl_user table has many NULLable columns without any DEFAULT (city, institution and so on...)
This cause a SQL error when the upgrade process try to set them to NOT NULL without setting any DEFAULT.
I suppose it is due to XMLDB introduced in 1.7, as other sites installed as 1.8 do not have this problem.
Probably the upgrade from 1.6 to XMLDB was faulty and subsequent upgrades did not fix it.
I already entered an issue in Tracker
Attached to the Issue there is a sql script to fix the NOT NULLs and DEFAULTs and bypass the upgrading errors
Actually, I was not able to test the upgrade to the end, as I had other problems after this; but this is another story...