When setting the stage for your players, keeping it short and sweet is the key. Imagine a pitch black room and a flashlight. You can only really see what's within the beam of the flashlight, right? Everything else in the room is shadowy and mysterious. It makes you wonder, it elicits emotions like curiosity or fear. Same thing with D&D. Players have a very limited "cone of vision" within your campaign world, so there's no point in beating them over the head with excessive information. There's also no point in designing things that your players cone of vision will probably never fall upon. Drop knowledge on them as they actively look for it, as it falls within their cone of vision, or if it's vital to the particular quest/story they're embarked on. Other information is best left shadowy and incomplete. It can be tailored and unleashed as needed. The important thing to remember is that your players control the cone of vision, you control what's inside of it.
If they don't care about the villains background, so be it. Save your
ultra cool story telling skills for that NPC or villain that they do
decide to take an interest in. Just get some of the important details in
there. Kinda like writing a thesis statement. "This bad guy is doing X
because of Y, and you should stop him because Z". That's all players need
to hear 95% of the time. Awesome treasure and stories obviously help drive the desire to slay the bad guys.
As for main story lines.. I feel that you should always have one, but it should take a back seat to
side quests. There are people out there who can explain this much
better than I can. My friends, I think, are more interested in running
short and sweet adventures that can be wrapped up quickly. Nobody likes dungeon crawls that become long and repetitive. Sometimes there's even room for two really short adventures in one session. A cool
way to do this is to feed off of what your players are interested in and
make short adventures focused around that, but that's a different topic.
- edit for one more little point...
Think about some video games you play like Skyrim, Fallout, Dragon Age,
any of the popular RPG titles. What are you doing 95% of the time?
Running silly ass side missions, kicking ass, and taking names. The main
story is still there. You're still aware of it because you're being
exposed to it in the background of what you're currently doing.
Eventually you move on from raping and pillaging short dungeons and you
start knocking out storyline quests. When you DM.. be a video game. Some
people will want to beat me to death for saying that, but I feel it's a format that works well for the casual Tabletop RPG'er. After all, we're in it to have fun, right? That means you, as the Dungeon Master, need to be flexible.