Limitations
There are many cool things yet to add to abakus. Here's a partial list of things that abakus doesn't support:
Complex numbers.
User-defined functions of more than one variable.
Unit analysis (for example: 3 A / 1.5 ohm -> 1.5 V)
Advanced input as in SpeedCrunch.
Numerical integration (finding the area under a given curve).
Graphing (? - I'll admit I'm not sure if this would be a great fit for abakus)
Matrices
Functions on lists (e.g. sin {0, pi / 2} -> {0, 1})
Session export/import (the session is still saved/loaded automatically).
More functions. Although many functions that aren't built-in can be simulated. For instance, to take the logarithm of x to a different base (b), you could do (ln x / ln b). And the xth root of a number is just that number raised to the (1 / x) power.
