Battery might not be an issue. HTC has done a good job of getting the most out of their smaller batteries on their last few devices.
If you're into rooting/romming at all, the S3 is second to the Nexus devices, and that's it. Been running an AOSP rom on my S3 for awhile now... the specs, plus the rom, make it my own lil' Nexus 4.
Edit: Yes the DNA's bootloader has already been unlocked, but if it's anything like previous devious, that'll be closed up by or right after release.