You can see the same mixture of old and new in the selection of applications. Trinity KDE is supposed to be able to coexist with KDE 4.x and run its applications, but Trinity KDE's own choice of applications are those designed to run with the KDE 3.x series -- and this limitation is probably the project's least appealing feature.
The trouble is, almost all of Trinity KDE's native apps are represented by versions two to three years old. Some of Trinity KDE native apps, such as digiKam, KMail, or Amarok, are not that far from approximating the latest KDE 4.4 apps, but they lack some of the features and stability of their KDE4 namesakes. When you do see a late model app, like KDE 3.2, it is a third-party app.
These native apps are the end results of long development cycles, so most people should find them adequate for general use. Still, they raise an interesting question: Is using older apps a fair trade off for using the desktop of your choice?
While some might agree, others might argue that this choice is a reversal of priorities, and the apps are what matters.
For that matter, is KDE 3.5.11's speed and stability worth doing without the innovations of the KDE 4 desktop, such as folder view, the semantic desktop, the social desktop, remotely run widgets, or arranging apps in tabs in the same window?
The question naturally arises, because the impression is that Trinity KDE is a long way from having such features. But, then, its supporters might not want it any other way.
Besides the question of how many people will want the KDE 3 series extended, Trinity KDE also faces a number of other challenges, as Sebastian Kügler points out.
To start with, the KDE 3 series depends on the Qt3 toolkit, which is no longer supported by TrollTech, the company that develops it, although documentation is still available online. That means that Trinity KDE is faced with either working with an outdated toolkit, or of taking on the large task of porting the KDE 3 series to Qt4.
After all, the rewriting of KDE with the 4.0 release was not just a whim of bored programmers. A large part of the reason for the rewrite was that KDE developers were frustrated by the limitations of Qt3.
Moreover, the reason people use major desktops like KDE is partly because of the extensive ecosystem of applications that spring up around them. To be a major desktop, Trinity KDE either needs to attract a large number of developers to support this ecosystem, or else convince projects that have embraced the KDE 4 series to also maintain KDE 3 versions. While neither task is impossible, both will require strong organizational skills to carry out. A project's community does not simply develop overnight.
None of these challenges is insurmountable, but collectively they do mean that Trinity KDE already has programming and organizational obstacles ahead. Personally, I view the KDE 3 series as good in its time but superseded, but Trinity KDE's mere existence is so quixotic that I find myself hoping that it will become a robust, living alternative, and not just a shambling shadow of KDE 3 hobbling along on limited contributions.
Besides, the free software world can always use a new desktop -- even if this one isn't altogether new.