I put in a bug report, and they pointed out that we're not suppose to copy the whole include directory from the earlier version, but just the config.php file. After doing so, it still wouldn't work, but with different errors. However, copying config.default.php to config.php and then changing the mysql, baseurl, spiderpassword, scramblekeys, &c. in it to match the 10.2 config worked. I'm up and running now, but still having one issue that I'll post a new thread for.
tl;dr - RTFM