Let's say you're Jane the chairmaker. I'm Joe the toaster maker.
Deflation is going on. So I, Joe, have some money saved up for when my chair breaks, but I know it'll be cheaper next year, so when it does, I just sit on the floor and wait.
Jane needs a new toaster, but she knows it'll be cheaper if she waits, so she sits in her chair and eats her stale bread.
Unfortunately, my boss at the toaster factory isn't selling enough toasters, even though he keeps lowering the price. So he lets me go. Jane's chair business isn't going well either, she can't seem to sell a single chair...
Do that for the whole economy, and things don't work well.
Now, there's a huge caveat, which is that we're assuming that growth/more stuff is always good. It's an important question in an era where we have the ability to wipe ourselves out/deplete our resources by just using too much stuff, at least in theory. But that's not the same debate at all.
Probably best to think of it this way: what is the economy (that is, trading goods and services with each other) FOR?
Even for the most die-hard hippy, there are benefits to trading with others who are better at their specialities, even if you're not talking about physical goods or using up resources (ie, hire a doctor to set your broken arm, hire a musician to entertain your friends). More of that kind of trading is usually universally considered a good thing, whether you're "keynesian" or not, because it means people are happier/healthier/enjoying life more. And if you think trading with others is good, then deflation is always going to be a bad thing, because it incentivizes NOT TRADING.
Lots of inflation is also bad. The exact level that's best is a matter of a lot of academic debate (the gold-standard people are basically arguing it should be zero, though they're pretty clearly going to end up with a deflationary scenario barring a way to make lots more gold), but there's no serious debate about deflation. It's always bad in the aggregate for a society.
-W