The most obvious is that this isn't an exhaustive compendium of the Gradius (and Salamander) lineage; there are some notable titles missing, such as Gradius Gaiden and Gradius V—a pair of titles which many feel are the best in the entire franchise.
There's no Gradius IV, either, and the handheld entries—such as Gradius Advance and the two monochrome Game Boy outings—are also absent. Finally, M2 had a chance to resurrect its WiiWare exclusive Gradius ReBirth here (now sadly unobtainable unless you resort to emulation), which feels like a huge missed opportunity.
Then there's the fact that this pack only includes the arcade versions of each game. As noted by some fans, the home ports—while lacking some content and generally being less impressive visually—were actually balanced better, and it would have been nice to see them included here.
Of course, Konami and M2 could argue that this was never intended to be the 'ultimate' Gradius collection; the 'Origins' in the title suggests as much. Also, in the past, Konami has divided franchises up into multiple packs—Castlevania has three such collections (Anniversary, Advance and Dominus) recently. Perhaps we'll get a second Gradius pack in the future?
Are you happy with Gradius Origins? Let us know with a comment.