(Disclaimer: mostly talking about graphics packs here in my post)
I admire the detail and underappreciated beauty of many tilesets, and respect the time put into them. I absolutely love sprite work. It's always been my favorite form of computer graphics, especially when done well.
However, I refuse to use them for Dwarf Fortress. For me they only add to my confusion at the best of times, and they really break my immersion from the game.
1) Confusion: Let's point out the obvious; tilesets often try to make things look like what it represents, but in doing so, causes weird situations where it's NOT what it's supposed to represent. This has already been covered earlier in this thread. It seems to me that I would just prefer a symbol that I will ALWAYS be able to instantly glance at and, according to context, be able to figure out. But when the tileset is TELLING me that something is something entirely different, it breaks immersion.
2) Immersion: For me, any game with graphics needs very appealing (read: aesthetically pleasing, not necessarily realistic) graphics to keep me immersed. Kingdom Hearts, with it's almost anime-3D style was appealing for its time, and kept me immersed. Mass Effect (sometimes 1, and sometimes 2), with its broad and epic tale of the future, had the graphics to match 99% of the time to keep me immersed. But tilesets? It's just not 'good enough'. There is a HUGE level of detail in the game for descriptions of appearances, inventories, equipment, actions, etc.
But with tilesets, all of that is lost. You're telling me that that blonde-haired elf sprite there is ACTUALLY bald, missing an arm and an eye, and bleeding to death? Don't buy it. It's far too literal. However, ASCII is immediately open for interpretation; it's not telling you that the little 'g' is a goblin swordsman with no nose and scars all over his arms, it gives you the bare minimum for your interpretation of details. If you want to know more, then you read a description. Imagination takes over. You're more immersed. In a way, I could cover all of this in a third category of 'accuracy vs abstraction', how sprites attempt to feign accuracy when they're not and ASCII only demonstrates abstraction, which it excels at.
That's how it is for me anyway. I wonder how the numbers of people who prefer ASCII to prefer graphics packs reflect the numbers of players with high levels of creativity/imagination vs those with low levels of it. Not meaning to be insulting, just a genuine thought, as I know all kinds of people do enjoy Dwarf Fortress. I'd never argue against having graphics packs, mind you, it's important that people who otherwise would never have played get to play, because this game is simply amazing. But graphics packs are just SO not for me; it's turned me off from a small handful of roguelikes that I've tried and otherwise would have liked.