I second the recommendation to hop over to the HalloweenTime SuperThread! There is an FAQ about MHP, plus lots of fun discussions about HalloweenTime at DLR.
http://www.disboards.com/showthread.php?t=3270919
In terms of costumes, you'll see a wide range. Everything from very professional looking costumes to the kinds you buy at the Halloween Super Store to t-shirts to no costumes at all. It's a good enough mix that you won't feel out of place regardless of what you wear. Just, as
Dot2Vegas said, remember that you'll be in DL, so plan accordingly (in other words, comfy shoes, layers in case it's warm/cool, etc.).
For trick-or-treating, I would wait on that until the second half of the party. The lines are pretty long initially, but taper down as the night progresses. Most of the candy stops are actually treat trails, so you can amass a pretty good stash of candy quickly. Most stations have similar things, but some have different items, so keep an eye out of less common treats.
The second Cavalcade tends to be less busy than the first. Fireworks are popular, so plan to spend a bit of time waiting for that if you want to see fireworks. Characters are scattered around the park, but the longest wait is usually for the villains and Jack Sparrow (assume he's there again this year). So if you want to see them, plan on spending time in line. Jack and Sally are normally out as well, but you can see them during the day (they are not party exclusive).
Eat before the party. You only have five hours to get in all of the MHP exclusive activities, so I wouldn't spend it eating (unless you grab a CS meal and eat while waiting in line or for fireworks/Cavalcade).