When I pulled my MacBook Pro out for its first field run at CES, one limitation I hadn't previously anticipated arose. And it's something you'll want to consider if you're a USB 3G user.
When I went to plug in my Verizon EV-DO card, all was well. But when I moved to hook up my digital camera's USB cable in the other port, my smile turned upside down—just like the old plastic MacBooks, there is almost zero clearance in between the two USB ports, meaning anything that's slightly above the normal plug size will block the other opening. Guhhhh.
On the old MBPs, there was a USB port on each side, for happier, double-connected times. Now if you use most any USB 3G card, you'll have to hang it limply from a short extension cable if you want to get at your other port.
Sure you can also buy a card reader for your ExpressCard slot (even though the CF cards found in most quality DSLRs require an ugly protrusion), but that doesn't help when you want to download images to your machine live via camera control software—a liveblogging essential. Also forget about using any USB card reader you had, certain bulky flashdrives or your prized USB Humping Dog without blocking the other port.
So if you're getting a MBP and a 3G card to go with it, go with ExpressCard for the 3G—a nagging little thing that the old MBPs had on this new version. Otherwise this thing is a dream, and our review is more spot-on every day. Just little things like this that you wouldn't normally think about that pop up. Have you ever been surprised by an unsuspected limitation of new gear like this? Let us know.