While mentioning the ten plagues, we spill out a drop of wine for each plague. In looking through various haggadot, I see that most say to refill the wine cups after this. I have heard that some do not replenish their cups at this time. What's at work here? Why do some people (including my family) not refill their wine cups at this time? We seem to be in a distinct minority.
When my brother was a little boy, and I was even littler, our dad asked us: "Why do we spill out drops of wine when we mention each of the ten plagues?"
"Because blood was spilled," my brother answered, and my dad approved.
While we rejoice at our salvation, we nonetheless retain our sensitivities to the suffering of the Egyptians by diminishing our joy, if only in the mildest extent. This may be why some people don't refill the cup, in order to drink a bit less wine, and thus reduce the enjoyment by that amount.
On the other hand, there is reason to refill the cup so that it should be full to the brim when we say the blessing over it. The prevailing custom is that the cup is refilled.