Before you upgrade from a 2.0.X version, please make sure you have the following!
1) Make sure your MySql version is 4.0.0 or higher!
2) Make sure your plugins are Compatible with 2.1!
Please check to see if ALL your plugins are listed over here:
If they are not listed there, please send an email to those plugin authors, asking them to upgrade their plugins! Also, if they are NOT listed on there, you might want to disable them! That's ONLY if they are giving you problems of course!
You shouldn't have any problems upgrading from 2.0.X to 2.1 if you really do have MySql 4.0.0 or higher. Perhaps one of your plugins are preventing you from upgrading!
More things can be added to this list, as time goes on..by Moderators and Admins of course!