(I'm told that many of my complaints herein are planned to be fixed, so I am more excited for the future than the tone of this post would perhaps indicate. Thanks to adamw and bowlofeggs for their feedback.)
Before one uses an item - be it a car, a piece of silicon, or even software - it is desirable to have assurances as to its quality. What assurances these are can vary depending on the part and use, but they are all some degree of "will this work as I expect it to when I use it".
Linux distributions (distros), roughly speaking, have two styles of release: periodic stable (where there are formal releases of the distribution, like Debian or RHEL/CentOS) and rolling release (where the package versions are always updating, like Gentoo). These stable-release distributions are always frozen (in some fashion) from an associated rolling release, be it another distro or a special branch for this purpose. (So Debian freezes from its unstable/testing branches, while Fedora freezes from its rawhide branch, which is in turn approximately re-frozen to become RHEL/CentOS.) Although these associated rolling distros (sub-distros?) are usually intended primarily for improving the quality of the main distros, they often become commonly-run in their own right (Fedora, Debian testing).
Of course there are quality assurance efforts involved in the freezing process, but I want to talk about the rolling efforts. Ultimately, we'd like some degree of assurance that package updates don't break (first) unrelated components on the system, (second) other packages that depend on it, and (third) expectations of direct users of the package. For each of these, there are automated tests that can be run (e.g., verifying dependencies, ABI compatibility checks, regression suites, etc.), and it is also desirable to have human verification as well. So let's look at some!
I will explicitly ignore the following distros: RHEL/CentOS, because I can't talk about it; Arch, because it's not documented anywhere; Gentoo, because the documentation isn't clear (I think it's wholly discretion of maintainer?); Ubuntu, because they do very little on top of Debian's branches; Mint, because it is three (!) steps of snapshot-and-freeze from a rolling distro; and any other distros I don't talk about due to marketshare. This does mean I can't talk about any purely rolling distros, which is unfortunate but so is their documentation.
So first, let's look at a distro I don't use: openSUSE. Their release process is documented here, and I'm also pulling information from here, and here. According to these documents, package updates start in Factory, which is rolling. As updates in Factory pass QA, they are moved into Tumbleweed. Tumbleweed is therefore also rolling, but with additional stability testing. Periodically, snapshots of Tumbleweed are taken and become beta versions of the next stable release (Leap). I don't have information on the details of the migration process, and as I have never used this distro, I cannot comment on its effectiveness.
Next, let's look at a distro that I contribute to and whose process I consider "working": Debian. The release process is documented here, here, here, and here. (Debian also has an LTS process that I will ignore for this discussion.) First, there is an optional repository - Experimental - which maintainers can use for potentially buggy updates that they do not wish to release to the general population. Normally though, package updates go into Unstable (permanently nicknamed "sid"), which is rolling. After an amount of time has passed (depending on the importance of the update; usually two weeks) and no new bugs have been reported as introduced by the update, it is automatically migrated into Testing. Every two years, Testing is frozen (i.e., no more automatic migrations from Unstable) for about six months, and becomes the new Stable release. (Whereupon migration into new Testing starts up again as normal.) Since there are users of both Testing and Unstable, this process functions well; Stable is extremely stable (which is the primary goal of the project), and there is a quality rolling release available in Testing (when not frozen).
And also, we have Fedora. An example of the release cycle is here, and further information is here and here. Fedora has a pure rolling branch called "rawhide" into which updates can be made at any time. Every six months or so, a snapshot of rawhide is taken and becomes the base for the next numbered Fedora release. Updates to non-rawhide branches can be made at any time provided they pass Bodhi (Bodhi is enabled shortly before alpha). Feodra policy encourages maintainers to provide "latest stable" versions of software, and so the releases tend to end up both with newer features and more bugs than Debian's more conservative packaging policies.
Bodhi is really what I want to talk about in this post. Bodhi aggregates views into many metrics, but package migrations happen solely on two criteria: number of positive karma (without too much nevative karma), or elapsed time. Notably absent from migration are requirements such as: packages in newer branches must have version (NVR) >= the same package in an older branch, package dependencies must make sense, and package updates cannot introduce new bugs. That's not to say these things aren't checked - they are or could easily be - but they are not enforced.
Here are some other problems with this approach.
In theory, updates which introduce bugs will be given enough negative karma that they fail bodhi, but in practice they are not - if a bug doesn't affect the user, they do not, in practice, provide negative karma.
Negative karma on updates is largely ignored by everyone involved. (Upon further discussion, this is contested by other members of the project; I'm not going to argue since it requires calling certain people out, and I don't want to do that here.)
Additionally, updates which fix bugs have a field for confirming that it did in fact fix the bug, but failure to do so is a migrataion blocker. (Following feedback from adamw, this is intentional since updates which don't fix bugs but don't break anything aren't harmful, though I don't agree since it delays fixing the actual bug, except in multibug cases where fixing one immediately is more important than fixing all later.)
Because tests are conducted by people who have binned themselves as "testers", they happen most on the newest branches of Fedora. Which doesn't reflect either the importance of the updates, or even where the users are, and results in non-latest branc updates just always migrating due to time.
To counteract this, Fedora - as part of the gamification efforts - has badges (think achievements) for testing updates, as well as a weekly leaderboard.
Choosing my words carefully, I think this leaderboard has killed the usefulness of Bodhi. (bowlofeggs points out that the existence of the leaderboard may not be the direct cause; we don't actually know the motivation of the people at the top.) The top five people on the leaderboard (and it only shows top five) have all submitted feedback on more than 1400 updates to the distro, across all branches. Looking somewhat arbitrarily at the person in the #3 spot, they've submitted almost 2000 feedback this week. If they spent 8 hours per day for five days a week doing nothing but testing, this would be nearly 50 updates per hour - a bit slower than an update per minute. If this were mostly failures of some kind, I could understand that - maybe they have an automated testing grid. But they aren't, and they don't. These five people are the names I always see submitting karma to my updates, and it's always positive. The only times I've in fact seen them submit non-positive updates were when dependencies were broken - which is automatically checked by Bodhi already. In the past three months of update submission (i.e. my last 12 submitted updates), I have received two useful pieces of feedback, and more than a third of those updates did not receive the requisite positive karma before time expired.
So much of what makes distros have a unique flavor is tied up in unwritten policy, but it's important to look at the explicit policy as well, especially for processes that happen at large scale such as update migrations. Fundamentally, there are some things that need to be hard enforced by tooling, if only so that maintainers (like me!) don't fat-finger and push the wrong thing, since if there are enough of us, inevitably someone will. And Bodhi doesn't do that. It places no virtually no requirements on the updates that I (or any other maintainer) can push to the distribution, no checks for my mistakes. And make mistakes I have: forgetting to include patches for the issues, most commonly, but I've also broken the dependencies of another package, and Bodhi frequently causes newer versions to appear in older Fedora branches if they get enough karma before the newer one does (though I maintain this isn't a constraint that I should be responsible for at that point).
I believe that Fedora does fill an important ecological niche for Linux distributions, and while I don't want it to become any other distro necessarily, I do think we need to re-evaluate how we're doing quality assurance.
(As a point of interest, adamw also linked me some bodhi 2.0 designs for comparison.)