While tilesets and graphics are totally arbitrary, and you should use whatever the hell you please, it has always annoyed me that the prevailing opinion seems to be that all new players should always use graphics sets.
I'm one of those people who honestly has trouble understanding graphics sets. The furniture symbols are nice, yes, but unless you are using a very high resolution font zoomed in a lot, most animals are a
lot less distinguishable than the ASCII (depending on graphics set, of course). I mean, yeah, elephants, crocodiles, etc are easy to distinguish, but all the brown quadrapeds? Meaning most domestic animals in your fort? All nearly identical, unless you want to squint and see the couple of pixels that distinguishes a dog face from a horse face. It ends up largely coming down to color, or the animal's pose. Sure, they look slightly different, and after you've k'ed it a few times you remember which is which, but how is that better than ASCII?
I also find that since they tend to be visually busy, with much less blackspace and less distinct symbols, I find them kinda tiring on the eyes, and they just blur together in a big mass of color, and I have a hard time grokking the whole screen, and I find myself squinting at individual tiles, even after I've familiarized myself with the tileset. Solid color inside floors are especially bad for this.
The only thing that really bothers me about default ASCII is the aspect ratio. Since it took me a while to figure out how to change tilesets, I eventually learned to layout square rooms in a non square font without counting tiles, but as soon as I figured it out I haven't looked back. I prefer square ASCII tilesets that are small, and I like diagonal walls, so I usually use
Dorten, but there are a number of similar nice tilesets, and I rather like CLA, as it solves most of ASCII's animal issues, though I don't like how it does floor tiles.
I'm not opposed to graphics sets, per se, I just think most of them try to fit too much detail into the tiles, so it can be difficult to distinguish them without inspecting color or pixels. I think that you could draw a happy medium between the rather sparse and abstract ASCII and the busy and less distinct full graphical representations of creatures. Perhaps a more symbolic but still meaningful style might be nice, like for a cat you could use something along the lines of this: