Yesterday evening I had the same problem.
Brand new iMacs (arrived 2 days ago) with Mavericks preinstalled. I had imported user folders from another machine (no programs or common files), set up Bootcamp, and installed the first programs (PS Elements 10, Lightroom 5 + VSCO Plugins, Spyder4Pro). Went into WIndows to install Spyder4Pro there as well, and when I returned, the machine was stuck in boot. Verbose mode, "pci pause: SDXC". HW diagnostics showed no problem, so after some research, I decided to restore my system from a time machine back up I had taken before installling Spyder4Pro (but after installing Elements and Lightroom). That helped.
Now, one could assume the Spyder4Pro software caused the issue somehow, but no. I installed it again, exactly the same way, and the problem did not reoccur.
On one hand, that's good (I like to calibrate my screen), on the other hand, it means the issue can happen anytime again, because basically, I have no clue what caused it. 😟
I googled a bit and it seems "PCI pause" is a device driver feature related to the hot-plug-capabilities of Thunderbolt. When a new device is plugged into Thunderbolt, it can happen that the OS needs to rearrange the address spaces assigned to the various PCIe device drivers, so it sends them all a request to pause while it does that. Of course I did not add any devices while booting, so the "pci pause" in question must be something that happens while the OS builds the very first PCIe device tree. Maybe (and these are just wild guesses) a bug in the SDXC device driver, or a sporadic HW fault of the SDXC reader, or something bad in the OS. Anyway, I strongly suspect reinstalling the OS just masks the bug and does not completely avoid it.
Of course, this would be a question for the actual developers. Is there any way to get them engaged to look into the issue?