One of the many things I love about the Dreamcast is how every special feature was a success. The web browser and DLC worked incredibly well. My old setup was a 15" tv sat on my bedroom desk like a computer monitor, with my Dreamcast to the side and a keyboard and mouse. I was able to use the browser to download/upload VMU files, games and animations with ease and even occasionally surfed the net. Also, online gaming was near flawless. You boot up the game, select online and you're on. So easy and well implemented.
The VMU was great, as I could transfer files with my friend at school and more than enough games utilized it, so it wasn't a wasted effort.
I cannot think of an add-on or peripheral that was a waste of space or time. The lightgun was great, the maracas were great, the mic was great for both seaman and online chat (Alien Front Online), steering wheel was great. The only accessories that were so-so were the ones that never left Japan, and I'm sure they were much better if you were living in Japan and could use them to their fullest (the karaoki add-on does not do a thing anymore due to needing to connect to a certain server and the DreamEye does very little without an internet connection and another DreamEye. Also, iChat and modern tech makes it useless). At least it's great for JSR custom graffiti. Take a DreamEye photo and you can use it in the game.
Compare all of that to some modern consoles. The Wii especially has had more failures than successes in terms of online play and peripherals. Dreamcast totally dominates it in those two fields.