I actually got curious about this as I thought about it a little more, and decided to check something. I'm sure what I've found is probably known to most seasoned vets here, but I'm still an ebook noob, so I was a little confused about something.
So, I downloaded the epub version of The Girl with the Dragon Tattoo from my kobo account, and de-drm'd it (shhh). I then opened it up in Sigil and started to poke around. In the fonts folder, the only font's embedded into the epub container were Arial variants.
looking at Styles CSS file, I noticed a discrepancy...all the arial fonts are defined, each under it's own @font-face, so that's good, but when you look at what fonts were actually supposed to be used in the books structure, it's mostly listing "Times New Roman", everything but quotes, which turns out to be the email exchanges in the books, and this was arial.
So, I grabbed the Times new roman fonts from windows and put them into the fonts folder of the container, then defined @font-face for it, pointing to that new file, and low and behold, the Kobo displayed them. Now, I can admit that I can't see huge differences between relatively similar fonts, so I imported another font and re-defined it to replace the Times New Roman...this is what I got
Now, again, I'm a newb, but this seems to me that the kobo can display embedded fonts well enough, but it really depends on if the book is coded properly or has the fonts it's looking for included with it in the first place...or am I missing something obvious?