I'm worried that granite (or some gray stone with a low melting point...) might melt and turn into lava, which might potentially harden into obsidian. That's a dangerous road to go down. That takes a lot of flow work and liquid bits for a result that I don't care about. I like fantasy lava the way it is!
Melting ice, now, that is pretty cool. It would be even more awesome if rivers on the world map ebbed and flowed based on snowmelt. Maybe they already do that...but eh, even in adventurer or dwarf mode if I go sit on the edge of a glacier for a year, I'd like to see a river pop out in the summer!
Melting stone walls I could care less. Won't happen except maybe for dragon breath, and that can have its own code.
As for sculpting chairs out of ice, I don't like that. It means that I the player will have to keep track of how hot my bedrooms are, and that's a real hassle. If I put a chair made of ice in the same room as even a coal forge, it had darn well better melt.
(OTOH, I'm REALLY looking forwards to building my fortress straight down on a glacier. Walls of ice are awesome. Having to build stone walls and a stone chimney for my forge would be even more awesome.)
Does the ground (ice) level of a glacier rise and fall over time? Is it possible for a fortress to get snowed in? That would be totally sweet. You'd have to dig yourself out. Wolves could even dig little dens in the snow... You could end up with a fortress entrance on the 4th floor with a stair going down to the bedrock, and as winter progresses, most of the stairway gets covered with ice until your entrance is on the ground level! Or you could build your entrance on the 1st floor, and in the middle of the winter, you'd need to rely on a *covered* ramp to get up to the top of the ice...
I think it's already been mentioned specifically, but having to make holes in the ice on a river/lake in order to get drinking water is awesome too.
Can hand axes cut through ice just like pick axes? Seems reasonable to me at least.