It's been a while since a step-change in functionality. Going from a plain eTrex to a mapping one was such, as was the upgrade from serial to USB with card storage (though I was always a bit lukewarm about the colour screen), along with the modern GPS chipsets of the 'H' models. The Etrex 30 was a marginal improvement over the Vista HCx: The improvements in the hardware design being offset by some dubious software decisions. The major gains there were a better bracket and the ability to read multiple map files - neither of which would justify the upgrade on their own.
I'm not sure what's coming next, beyond the usual Moore's Law stuff. Marginally useful cloudy wireless bollocks, I suspect.
What I'm really looking forward to are e-ink displays. No sign of those on the horizon yet, though.