Well, to be fair, Hydaenlyn never commanded or even requested Minfilia sacrifice herself. The only thing she told Minfilia to do was run back into Flow... but again, that comes back to the question of why Hydaelyn issued that command in the first place.
The sequence of events goes like this:
1. Thancred and Y'shtola fight Crystal Braves in the Belah'dian waterway.
2. Y'shtola casts Flow.
3. Hydaelyn tells Minfilia to run back into Flow.
4. Thancred is warped to the Dravanian Forelands, Y'shtola is lost in the Lifestream, and Minfilia is stranded in the Aetherial Sea.
5. Minfilia prays to Hydaelyn.
6. Hydaelyn assimilates Minfilia.
The big, big question is: why was 3 above commanded? It was completely unnecessary at that point: Thancred and Y'shtola were already caught up in Flow and Minfilia was in no danger. I don't want to believe Hydaelyn called Minfilia for the sole purpose of assimilating her (and thus extending Her time) or using her as an avatar, but I can't think of another conclusion. Even if it's an act of desperation... that is not okay.
Back to the cultural argument: it probably is a big deal, with the collectivist vs. individualist mindsets clashing (Eastern ideals are generally more collectivist, while the inverse is true in the West). I'm a bit late to the party, but to further augment the discussion, in Magic: The Gathering philosophy Eastern ideals, being collectivist, tend towards Selesnya or Bant, while Western individualist ideals tend towards Izzet or Grixis. Anyone who knows those terms knows they're bound to clash. I'd explain more, but... I don't want to. Maybe on request or when I'm less tired, but it's a long and nerdy philosophy dump.
As to whether or not it's okay to sacrifice one person for the world... that's up to you. It's easy to say "Yeah, sure" when it's somebody you don't care about - but everyone cares about Minfilia, and this has hit them hard. The Warrior of Light is getting rather melancholy with all the sacrifices that keep adding up, Y'shtola is being unusually firm and missing Matoya's point entirely, Thancred is acting uncharacteristically cold and brusque, and Alphinaud is desperate to prove to himself that we can save someone close to us (to the point that yes, he intends to defy the Gods if need be). Regardless, numerous questions remain such as why this was necessary, and whether or not everything we've done has made any real difference.
Really, it comes down to a very basic choice between these two lines...

Originally Posted by
Duke Pantarei
The future must be protected.

Originally Posted by
Yuri Lowell
We make the future! You trust the path you choose and create it.
I'm pretty sure I know what Hydaelyn wants us to seek, though...
Souls. Larger, more powerful souls.