So why do I like Seras? In two words: she's fun. In more than two words:
Seras is fun to draw. I love all the tiny little details on her uniform, and she's the character with the most diversity in her wardrobe, which means she comes with built-in visual variety (as opposed to, say, Walter, whose closet is filled with a dozen identical shirts, vests, and pants). And then there's her hair. The spikiness is sheer entertainment.
Seras is fun to throw into comedy. Her sweetness and innocence combined with cuteness makes anything she says funnier, simply because it's her. (Cuteness is one of Scott Adams' six dimensions of humor. You need at least two for amusement; Seras has one by default.)
Seras is fun to pair with Pip. Now, Pip is a lot of fun all by himself; but throw in Seras - whose innocence contrasts brilliantly with his perviness - and you have a recipe for endless comic hijinks. Seras will actually stand up to Pip (in a way that she won't to Alucard), so, to steal a phrase from Bill Watterson, the two personalities just bounce off of each other. The developing AxI is strained with tension; the developing PxS is riddled with amusement.
Seras is fun to write, period. Kōta Hisrano has said that she's his favorite character, because she's the "light that stands out from the darkness": the good and sweet girl amidst a crowd of jaded and ruthless characters.
Seras would be fun to be with. Again, she stands out from the rest of the crowd: most of the characters are very cool, but meeting them in person would be frightening. Seras is one of the few who's mild and unintimidating enough (at least at first) that one could be comfortable actually hanging out with her.
Finally, Seras' guns are fun. Mostly because they're bigger than she is.