What's so great about the bitterness? Why do we want to remermber  that ?

Actually, our bitterness in Egypt was/is the key to our redemption. We never got used to Egypt. We never felt we belonged there. We never said, “They are the masters and we are the slaves and that’s the way it is.” It always remained something we felt bitter about, something that was unjust and needed to change.

If it hadn’t been that way, we probably would never have left. In fact, tradition tells us that 80% of the Jews said, “This is our land. How can we leave it?” And they stayed and died there.

But as for the rest of us, when Moses came and told us we were going to leave, we believed him. It was our bitterness that had preserved our faith.
Everyone has his Egypt. You’ve got to know who you are and what are your limitations. But heaven forbid to make peace with them. The soul within you knows no limits.

This is the sweetness we apply to the bitter herb: Bitterness alone, without any direction, is self-destructive. Inject some life and optimism into it, and it becomes the springboard to freedom.

haggadah Section: Maror
Source: http://www.chabad.org/library/howto/wizard_cdo/aid/117120/jewish/9-Bitter.htm