So, you think that appreciating stock value should be taxed but not appreciating real estate?
I don't see a conceptual problem with taxing gains that are effectively realized (e.g., by taking a loan against the underlying appreciated property, rather than selling it). This would include HELOCs, which have already been limited to a substantial degree.
Details of rates, threshholds, and exemptions can be haggled over...
Secondly, would you favor allowing people to deduct decreases in stock value on their taxes?
If you'd like to suggest a mechanism by which decreases in the value of stock that is not sold can cause outward cashflow I'll consider it. (Although I did mention above the possibility of getting around the subject problem by having certain large shareholders and/or insiders adopt the mark-to-market rules, which would in fact allow them to apply unrealized losses against unrealized gains, even, I suppose, to the point of showing a loss on their tax return.)
ETA - I suppose I should have said this "could" include HELOCs. Borrowing against equity that was created by paying down a loan is not the same (I think) as borrowing against equity created due to an increase in the home's value above its initial purchase price...