This collections is meaty, varied, variously fun and clever.
There are so many good stories that listing highlights seems a bit futile, but for me these would be Ian McDonald's surreal high concept "Verthandi's Ring", Tony Daniel's semi-fantastical, far-future romance "The Valley of the Gardens", Alastair Reynold's "Minla's Flowers", a decades-spanning tragedy with a strangely Lilliputian feel, Robert Silverberg's addictive and perceptive "The Emperor and the Maula", and the excellently paced, caddish romp "Send Them Flowers" by Walter Jon Williams, whose the character who at first might be taken for the comic relief, Tonio, quickly steals the show.
"Muse of Fire" by Dan Simmons deserves special attention. It features an old-fashioned travelling Shakespearian troupe somehow surviving in an empire of human slaves devoid of all other culture and stripped of their homeworld, as they climb a hierarchy of alien powers to perform a series of the Bard's works with escalating importance to the future. It walks the line between ridiculous and sublime with absolute and expert poise, and serves beautifully as a climactic championing of the human spirit in a science fiction world, and a defence of great poetry in the broadest sense.
I liked the plot but ultimately found the mother in Nancy Kress's "Art of War" offputtingly one-dimensional. Stephen Baxter's "Remembrance" was good and will probably mean more to those who've read the Xeelee series (it's early in that timeline, apparently).
Obviously there has been science fiction in short story form for a very long time. However, I tend to think of the label "space opera" as applying to a longer format: novels, or multiply franchised TV series, for example. An opera is not a single musical piece, after all. This collection really demonstrates that the epic tale, "told on an enormous stage" as the Introduction puts it, can fit just as nicely into a short story, thankyouverymuch.
In fact, it did lead me to thinking that in many ways the short form can encapsulate an epic view while cutting down on long-form description which can sometimes drag (as I often find in "old space opera" novels).
It might just be me, but I also think the short form for space opera is best suited to something in between the serious and the fun. While I love the darkness of Iain M Banks' SF novels, for example, here I found the murder and moral wrangling of Gwyneth Jones' "Saving Tiamaat" (the opening story) and the bleakness of Robert Reed's "Hatch" a little unmoving. And on the other hand I didn't find Kage Baker's cutesie "Maelstrom" very funny, personally, (and it's theatrical theme is clearly outdone by the climactic "Muse of Fire"). I wonder if the short form, in space opera, doesn't have time to develop a really dark feeling, and on the other hand feels throw-away if the intention is merely a cosy, comedic character study. Maybe getting somewhere in the middle is what makes the short form so difficult, especially in science fiction: you need to avoid being too trivial, and on the other hand to establish your "operatic" feel without undermining it with the short length.
All in all a highly recommended collection and the presence of lots of big series authors makes it a great pointer to who to read next.