Just a quick point of clarification. WordPress 2.2 is a mandatory security upgrade for WordPress 2.1.3 users. There will be no further releases in the 2.1.x branch. 2.2 includes security fixes that 2.1.3 lacks, so your only upgrade path is to 2.2.
The good news is that although a lot of under-the-surface code changed in 2.2, it’s not jarringly different from a usage standpoint. So if familiarity was holding you back from upgrading, it shouldn’t be.