I'd like to note that I did actually pull out my Dreamcast to make sure my comparisons were fair (being a fan, it wasn't exactly hard to motivate myself to do this). I had played most of these games when they originally came out, but of course, that was about a decade ago. I can still say the Dreamcast looked damn good for its time, and it has aged rather gracefully for that amount of time.
The most obvious enhancement culprit is Sonic Adventure. Dreamcast Collection has rather obviously updated Sonic's model to a crisper, shinier one. Strangely, Sonic seems to lose some of his shine later in the game, and looks more like the original, matte Sonic. After this point, I'd say the improvements to the look of the game are actual improvements, but shiny Sonic (and shiny townsfolk and shiny just about everything, for that matter) is pretty annoying for me. Adding shine to everything indiscriminately does not make a game "newer" or better looking.
Sound and music seem to be slightly different as well, but the most noticeable game on this matter is Crazy Taxi. As you may have heard in passing, now the game is stripped of its original Offspring soundtrack, and replaced with what could be some old man mumbling the words to a Creed song for all I care. Ok, the background music is now a generic rock track, just to explain for this review's sake, but it's just not the same. Beyond that, however, the sound effects are just off. Honks from passing cars seem annoyingly loud and repetitive. Taxi patrons sometimes sound like they're speaking from a tunnel. Worst of all, your car now makes the loudest, most annoying, tinny scraping sound anytime it touches anything. It was there in the original, but it seems tuned way up in this port to just plain disrespectful levels.