Wow, you asked a really good question! I've traveled solo to both WDW and DL, and have had a magical time! Once I even traveled solo on my birthday, and stopped by City Hall on my way in to get an "It's my birthday!" pin. Once I put it on, people kept wishing me happy birthday wherever I went throughout the parks! I just usually said, "Thank you!" and let it go at that, but some stopped me and asked me a few questions about where I was from, etc. While walking to Fantasyland from Tomorrowland, the 4 sax players saw my pin and started playing "Happy birthday" and got a total and complete stranger to sing it to me along with the music! It was magical! One of the big perks of going by yourself is that you don't have to wait in stand-by lines -- you can usually walk up to the attraction and walk in the single rider line (do they still have those?) and get on almost everything as if you had unlimited fastpasses, depending on what time of the year it is. I'll tell you, it might seem lonely in the beginning, but the more you do, the less bored/lonely you'll feel.
What you could also do is see how many people you can get together on Disboards to meet and have coffee with you somewhere in the World. I did this on the same trip, and I met a wonderful woman whom I'd been corresponding with for a couple of years online, but never met, and along with her husband, they took me to Kona Cafe and they bought me dinner and a piece of birthday cake! So it's only how you look at your trip as a single traveler. If you WANT it to be lonely, it will be. If you make the most of it, you'll speak to lots of people on any given day.
Just as an FYI, I'm traveling solo for about my 4th time to WDW at the end of April for 8 days. When are you going?
Dan