A polarizer cannot be duplicated, it is a must have for outdoor shooting.
Infrared doesn't matter on a digital camera usually, because of IR blocking filters over the CCD.
Most digital-editing filters are nearly-impossible to distinguish from actual lens filters.
I sold my chroma-enhancing, stars, NDs, softs, etc. quite a while ago, as I see no use for them.
Judging by the turntable-aficionado resurgence, stay tuned for the anti-digital photography backlash to come, however.