brilliant game, you have totally captured why we do dig games. being able to choose what kind of dig game, as you upgrade, is amazing. short of the flash classic 'Go To Hell' it's my new favorite. your pacing is phenomenal. (i would suggest being able to turn off auto-mode in that final explosive upgrade.)
however, the one thing that's going to keep this a webtoy instead of a popular sensation is: your item choice. not your art - your art style would be fine even out of prototype. but dudes, your inventory is boring. it needs to be surprising. FUN. every item should be legible, yes, but more importantly we should go LOL every time. and as you upgrade, LOL OMG. maxing out the 'rarity' stat should not have been this underwhelming. we commend you for labeling everything however 1) we shouldnt be depending on the label to recognize items, 2) those labels can add extra delight and humor to the game.
items that would be *silly* if you dug them up. shiny, too, though every dig game does shiny. it does not have to be as bizarre as Kingdom of Loathing, but you need a Pokemon sort of artistic hook. otherwise the only hit of serotonin is from watching the money counter - and that is a bit soul-less, isn't it? we would highly recommend consulting a full-time pixel artist.
also i'm sure someone's said something about making soil layers but that's secondary to the gameplay - and i like your simple color scheme! other dig games are eyesores with their gaudy terrain types. the simplicity also vital to your sense of movement - which you've also done very well.