Ors, that's a strangely shaped bag (I prefer thinner profiles as they "bounce around" less. Here's a few suggestions; not necessarily for specific equipment, just learned along the way.

1) Don't make it TOO big. In fact, your wife may be going the wrong way with saying that bag is too small.
2) Multiple pockets accessible externally or compartmentalized internally - though less desireable - is really important.
3) At least one bottle holder outside with cinch straps or elastic. The latter is easier access, though spillable if not strong. 2 holders isn't a bad thing either.
4) It will get dropped, yanked, pulled, drug along, kicked, spilled on, and otherwise abused. Make sure it's quality, but light material.
5) Make sure it has a comfortable shoulder strap. The strap should have rubber grips. A backpack isn't amiss.
6) Get at least 2 bags. One is always ready to go for quick trips to town and shouldn't have much beyond the basics, and only doubled up on any one thing (diapers can be trippled). The bigger bag is for long trips or outtings. Surprisingly, newborns actually require less, and the more you have the carry around, the less fun (so you're purchase probably isn't a bad thing!)