Samsung's one of the few manufacturers who don't try to lockdown their devices in every way possible. The bootloader on almost every recent Samsung device has been easily unlocked, and all have been rooted relatively easy. It looks as if that trend will continue with the Galaxy S4, which is great news. We can't speak for the bootlader just yet, but the device has already been rooted.
One of the wonderful developers at XDA has obtained root access on a Galaxy S4 I9500 ahead of its public release. Don't get too excited though, as it looks like this process only works on the I9500, which is the Exynos 5 Octa powered device. Initially, most counties are expected to only get the Snapdragon 600 powered variant. We wouldn't be surprised in the least bit to see the Snapdragon 600 model rooted relatively soon too, though.
Rooting is a process that allows you to perform unofficial modifications to your device, though some can be a tad buggy. It's important to note that simply having a rooted device does not allow you to install custom the ROMs that you may hear of. If past history is anything to go off of, then we can expect the Galaxy S4's bootloader to be unlocked relatively soon after its release.
As far as the openness of the U.S. Galaxy S4 goes, we can expect the T-Mobile, AT&T, and Sprint variants to be relatively developer-friendly. The Verizon model is a different story, though. The carrier has not been developer-friendly over the last couple of years, so if you're a person who likes to mod your phone, one of the other U.S. carriers will be a better option.
The standard disclaimer applies here, though. Android Headlines is not responsible for any issues caused by rooting your device. If you perform the process correctly, then you shouldn't run into any issues, but you never know. Especially with a product that is in the early stages of its life.
We'll be sure to keep you updated as to other Galaxy S4 developments. It will most likely be much easier to mod than the HTC One, but we'll just have to wait and see. Check out more about the Galaxy S4 root process 0n XDA.