From this screenie, it should be obvious that KDE's new toolbar default of "Text Under Icons" does not work unless one has a screen with 5,000 x 5,000 resolution.
which nearly misses the point. it is very true that many of our toolbars when used with text under the icons take up a lot of screen space. however the problem is not bigger toolbar buttons, which are good for usability in many ways (more on that in a second). the real problem is that we cram so many (often stupid) things into our toolbars.
so what are toolbars for? toolbars give rapid access to the key actions in a given context (which is usually a task-specific window).
toolbars are not a billboard to advertise "neat-o" but rarely used features. that's what menubars are for. so when "mail" appears in a painting program's toolbar (kolourpaint) one really ought to say themselves, "wait a minute! this isn't an email program! this is a painting program! emailing a doodle isn't in the top 5% of actions! it's not a core part of this task's workflow!" and for most apps, even the top 5% of actions is going to be more than you want to shove in a toolbar.
another tip: if your toolbar icon's text is more than 2 short words long, ask yourself why you're being so wordy. probably it's because the action is ill defined or you're trying to be overly exact. remember that users can mouse over for tooltips and that once they learn the toolbar (given that it's learnable, e.g. less than 10 or so items on it) they won't rely on the verbage anyways: it becomes a matter of muscle and visual memory (placement and iconography). that's right, i'm looking at you "next unread message" in kmail and "mark feed as read" in akregator! why not just "read next" and "mark as read"?
in other words, changing the toolbar defaults is meant to make application developers rethink their abuse of the user experience through really poorly thought out toolbar entries.
i mentioned earlier that bigger buttons in the toolbars are better for usability. why? bigger objects are easier to point at. bigger objects also mean bigger icons which means greater clarity (22px icons are pretty hard to make really clear, particularly given the specificity of some of our actions). bigger objects mean fewer objects which means less clutter and greater learnability.
so rather than bemoan text-under-icons, perhaps take it as an opportunity to make your toolbars suck less(tm). users of kde4 will thank you. chances are you'll thank yourself.