The only sold out day (so far) is Saturday; if they have 3-day tickets left you can still purchase for Saturday entry.
Have you ever been to an entertainment convention before? It's very similar to Comic Con and others. Lots of lines, lots of waiting, but lots of fun. If you're interested in just walking around and taking everything in, your best bet may be to visit in the afternoon. Lines tend to form around 6 PM for the next day's events, and usually don't wind down until 11 AM - 12 PM when everyone is slowly let inside. That said, Expo rules change from year to year, and we haven't heard any updates on overnight queuing or line formation limits.
There are things to do beyond the big panels and smaller, limited attendance panels: 3rd party art and merchandise shopping, exhibit walkthroughs, and some floor shows. I'd check out the schedule and see if anything interests you before you commit.