Depends on your definition of "stable". Since distros vary widely on what they consider to be "stable", comparing various distros' stable releases is like comparing processors by raw GHz values or dSLRs by megapixels alone.
Debian Stable is the last place to go if you want very recent builds of packages, but the first place to go if you want absolute, rock-solid stability.
Debian Sid (unstable) has the latest versions of each of the packages, though Debian's guidelines are strict enough that testing and sid are oftentimes more stable than the "stable" releases of other distros.
My advice (and this is what I do): Run Debian stable (Jessie, as of today) as a base image, and use an appropriate container for applications that require more up-to-date applications. Best of both worlds.
Since the freeze is around 6 months, this means you get 6 months old software when Debian is released. There are some exceptions, like browsers that are too difficult to maintain at the same version.
We believe most people like this definition. This can be frustrating when you need the latest version of nginx but you are happy that upgrading some basic stuff won't break anything on your system: no deprecated configuration option in X, no command-line flag that doesn't exist anymore in Y. All should work exactly as before, with fewer security holes and bugs at each upgrade.
However, if you really want to have the latest version of a selected set of software, have a look at the official Debian backports. This is a great strength of Debian over Ubuntu (where backports are almost inexistant with the notable exception of the kernels): there are many backported packages. For example, if you need a more recent version of nginx and you are running Debian Wheezy, you'll get nginx 1.2.1. If you need something more recent (because you want to get SPDY), you can get nginx 1.6.2 through backports. See here: https://tracker.debian.org/pkg/nginx.
Backports are packaged from the versions that will be in the next Debian release. So, they should keep the same quality than the packages which are currently in Debian. This is a great strength over random PPA. Some of them are maintained by skillful people, some others are not. If you trust Debian for its packages, the backports are made by Debian Developers too.
For nginx, there is no 1.8 because backports are taken from the next release. As this next release is currently frozen, the version proposed in backports is still 1.6.2.
Using a Debian Stable with backports should allow you to get what you want: stability for most packages but latest releases (and latest bugs/changes) for a selection of packages.
If you can identify the specific packages (i.e. php? nodejs? mysql? redis? etc) that you found outdated in Ubuntu it will be much easier to make a suggestion about how appropriate Debian stable will be for you.
I can't say why that is, but I've seen it on multiple occasions. Usually you can resolve this in Ubuntu with a PPA.
Because Ubuntu releases are, well, releases. Early in the release cycle, they sync from Debian, they package their own things, then freeze the versions and release. They don't change versions of most packages in released! releases.
Debian unstable on the other hand, has no concept of a release, so maintainers upload new versions of packages into unstable pretty much all the time (except freeze time).
So what you've seen is actually the norm, not exception.
In Debian, actually, I am pretty sure there's never really a freeze in unstable as you mention. They make a new stable release from the testing branch some time (a good long while) after the freeze is called, then for a brief period you have only stable and unstable (and oldstable), and later on a new testing branch is created (not yet frozen) with whatever packages from unstable meet the criteria to go into testing.
When testing is not frozen, that means "the package has been in unstable for 2 weeks without any reported bugs" or something like that. When they're getting ready for a new release, they freeze testing, and then the criteria to get your package in for the next release gets more stringent (is it, bugfixes only? security issues only? I'm not sure, but it's probably even stricter than I think.)
I'm not closely following Ubuntu development, but they have 6 month release cycles and they do the "sync from sid" early in the cycle, AFAIK first 3 months or so, so you can expect 15.10 to have what was in sid on August-ish. This may further skew when
1. Debian is on freeze. For example, Debian was on freeze starting from November '14, which means practically sid was also on freeze. Debian import freeze date for Ubuntu 15.04 was February [0], so while normally you would expect 15.04 to have latest versions of february; in reality they were latest versions of october. (there are, of course, exceptions). When you consider import freeze of 14.10 was August '14 [1], you can see why 14.10 and 15.04 have very similar versions.
2. Ubuntu is importing from testing for LTS releases, instead of sid. This shouldn't matter in an ideal world, where the difference between testing and unstable is 5-10 days, but sometimes packages got stuck in sid so bad that it may cause a difference in what lands in Ubuntu release.
3. I'm not so sure on that but if I understand correctly, they do a complete import of Debian in the beginning of the release cycle, and then maintainers can do ad-hoc imports for individual packages until the import freeze date, so the packages that land in the release may also be older than what was on sid near the import freeze date.
I guess you'll see much newer packages on 15.10, though, since sid will be full-speed during the 15.10 cycle, so nothing to worry about for now :)
Thanks!