It's a pure Reverse Polish Notation calculator with no algebraic options to confuse the user.
Four stack layers # Odd period calculations (allowing interest to accrue for some arbitrary amount of time before the first payment.)
Bond calculations
Depreciation calculations
Improved action of SST and BST features
A much larger manual with useful chapters on real estate, lending, investing, leasing, savings and bonds
Uses three SR44 batteries (included)
Can run for months or years on the same set of batteries (The record so far is 17 years on the original set of batteries and that sample is still running at HP's Labs. Actual battery life varies with use.)
Built in self test
First introduced in 1981, the 12C is still one of HP's most popular calculators.