Same as the above, don't read if you care about Supernatural spoilers.
I can agree that Ruby's actual death was anti-climatic (apart from the fact that it was -her- who gave the brothers the means to kill her in the first place, so we get some irony or whatever). It's the few minutes she was alive between Lilith's death and her own that made the scene so fascinating.
She wasn't evil in the same way most of the other demons were. She didn't care about human lives, and her goal was to revive Lucifer, but she didn't seem to be doing that out of malice. She very clearly was a parallel to Anna's own thoughts about angelic nature. Lucifer being originally an angel who 'fell' like Anna. Real Paradise Lost stuff.
And she spelled out her motives to Sam so clearly, too. She really had no intention of hurting the brothers, she basically idolized Sam and it took everything in her to keep her secret. She herself seemed surprised she could do it. But once it finally happened, she could let it out, she could finally show Sam her true self and now she had a chance to 'convert' him openly. She talks to him in almost a motherly tone, telling him it's okay, they're just finally letting a fallen angel out of its cage. Things will be good now. Keeping in mind the idea that most demons are just humans who have survived hell long enough to become twisted, the other 'good' demons we met in the previous season explained that they just don't want to be in hell, and they'll do anything to get out and stay out. From the perspective of someone who sees Lucifer as just another fallen soul being tortured in hell for the simple sin of disobediance (and later, rebellion, but this is from Ruby's perspective), she's saving her sympathetic 'father'.
It's really not that much different from the general theme of Supernatural: Stick by your family at all costs.
I think the fact that she was so devoted to Sam that she would turn her back on him to face Dean (thus causing her untimely demise) is pretty telling, too.
Explaining it this way feels like I'm cheapening it. Genevieve's performance in this scene is what really sold it and completely changed my opinion of her as an actress and of Ruby as a character.
Out of curiosity for Season 5... without giving anything else away... have any of Jo, Ellen, other major characters from past seasons, shown up at all? Or are they basically forgotten loose ends?