A few random thoughts and observations:
-- Clearly there is no "penalty" just for changing titles. One needs to understand what a penalty is; that term gets thrown around far too much.
-- What does seem apparent is that changing titles calls into question the trust put into those pages. I didn't say it affects rankings, necessarily. But knowing that algo's have a lot to do with assessing probabilities (e.g., of spam), title changes seem to have fallen into a category that can potentially be a predictor of spam at some level of confidence. This almost certainly has to do with the next two points:
-- Frequent tweaking is a problem almost certainly. Seen examples.
-- Larger, higher trust sites, seem largely immune.
Question: When do title changes (frequent, sitewide or other) also call into question the trust of the site, and/or affect a site's ability to rank for certain kw's?
-- I've seen some evidence that changes to a significant number of pages on a site can affect sitewide rankings. It was only a few years ago that people still scoffed at sitewide rankings measures and actions, but that is not really in question anymore. Some factors do affect G's perception of an entire site and given G's exceptional paranoia about spam these days, it seems at least a reasonable supposition that large scale (e.g., sitewide) page title changes might be taken not too differently than frequent changes are taken: As a sign of undesirable SEO activity.
-- When title changes do seem to be the culprit, time to recover varies widely. I know one site that's waited 3 months to recover, so please don't tell me it doesn't happen. (Only change: Sitewide page title changes, that affected rankings on core and related terms. Wasn't a case that some rankings went up while others went down...they all went down.)
-- The contention that the brand name should never be first in the page title may apply to most smaller websites, but it is a poor conclusion to offer as an absolute, IMO. While I can agree that the default approach might best be keywords first, it's not the only good approach. Especially in the case of trusted brands, sometimes the brand name upfront significantly improves CT's in the organic SERP's and if one's site is already kicking butt in the rankings, the presence of the brand name upfront can make significant differences for the better in overall revenues, given the differences in click rates. Again, helps to see it in action. ;-)