Runtime packers are pretty much viruses that are compressed into zip packages or rars, or 7z, or whatever, and release when activated.
There was big talks about this at Blackhat.
Blogs on eset as well : hxxp://www.welivesecurity.com/2008/10/27/an-introduction-to-packers/
Why its not for default could have to do with system resources, ensuring weaker computer systems dont get bogged down and freeze up.
Smart optimization kind of falls in a similar category, it basically speeds up the scans, which will help detect different sources of malware, but also might cause others to be skipped over, so they chose off by default.
Long thorough scans are a more proven method in finding files, which scans every file on the drives & system, not just the high priority targets.
Timestamp is an option to keep the original access time of scanned files instead of updating it , and is more or less for backups, and files that have been altered or zipped, or what have you ....
Off by default because not everyone uses backup or similar and the files dont really change in size or format etc.
Thanks for explaining all that, but how about the new features in the HIPS?