@Sousa
The spine of JSA (and I expect GL given that DC Omnibi have followed this trend for a while now) is slightly curved, but ultimately the shape of the spine itself isn't all that important to the binding. The recent Saga Omnibus by Image has a completely flat spine, as do Dark Horse's Library editions of Hellboy and they have some of the best binding around. Binding is why most people love omnibus editions. Good binding is make or break. Glued is blasphemy, Sewn is the standard by which all stand by.
The quality of sewn binding depends upon the flexibility of the fabric strip the pages are sewn into. This usually depends on how much and/or what kind of glue they have used to strengthen the book (as those things are heavy!). Rigid glue means that despite having sewn binding, that fabric part will not bend very well when the book is open, stopping it from forming the 'curve' or 'triangle' that lets you see all the way to the middle of pages. Glued binding has the pages glued to the spine itself instead of letting it bend freely, causing terrible 'gutter loss' and you lose art in that giant crevase.
Marvel Omnibi are always sewn (with the exception of their very early attempts) and have always been very good with the flexiblity of their binding whereas DC have very slowly been improving theirs and are basically playing catch-up. While they do use sewn binding on their new Omnibi, the glue they tend to use to strengthen it makes it more rigid than people would like, which is what people mean if you hear the word 'mousetrap'. They're referring to the way the glue prevents the fabric from bending and thus the middle of the pages won't rise to be level (flat) with the ends of the pages.. this also comes with the problem of the book not liking to stay open if you're not at the middle of the book.. thus swinging closed like a mousetrap.
Ultimately when you're buying non-Marvel Omnibus, I'd check on here for information on how good the binding is as every volume seems to vary. DC produced a fantastic binding with their Sandman Omnibus Vol 1 some time back and then surprised many with a downgrade of quality with their Vol 2 and JSA Vol 1. However, JSA Vol 2, which is their most recent Omnibus I believe, has the very best DC sewn binding I have ever seen, it's almost on par with Marvel's early sewn binding, just as good as the Walter Simonson Thor Omnibus I'd say. So everyone is crossing their fingers with the Green Lantern Omnibus Volume 1.
Hope this helps!
Mousetrap DC binding of the past
http://i208.photobucket.com/albums/b...r/IMG_0073.jpg
JSA Omnibus Vol 2 DC Binding of today
https://fbcdn-sphotos-b-a.akamaihd.n...47607417_o.jpg
Note that beautiful curve that lets you see the middle of the pages with ease. The binding didn't need any loosening for JSA Vol 2, it was like that from the beginning. DC is really making strides.