Kate
The Kate editor came bundled with my SUSE KDE desktop, and offers a more technically robust text-editing experience than, say, WordPad on Windows XP.
Alas, my first impression was extreme annoyance with a dot that showed up where a space should be when I hit the space bar after typing a word. Perhaps this is useful for certain types of programming, but it's a major distraction when you're writing prose; it looked like there was a period in the middle of my sentence when I paused to think and look at what I was doing. I found a configuration setting in the program to "remove trailing spaces," but nothing about those blasted dots.
Eventually, I found an answer by searching through archives of a KDE mailing list: Go to the editing section of the configuration menu, and under tabulators, uncheck "show tabs." I am quite sure I never would have guessed that one!
Kate offers a lot of conventional text manipulation, such as search (including regular expression support), replace, change text case, and join or split lines, as well as spell check. However, there's no built-in support for making text bold or italic, and changing fonts requires going into a Configure Settings menu (as opposed to using a tool bar).
Kate doesn't come with HTML coding support switched on, either. An HTML plug-in is available, but it comes with no documentation. The plug-in offers some basic HTML syntax highlighting if you create a file with an .html extension, but I didn't find any easy tools for doing HTML tasks such as inserting hyperlinks or bold tags. There was a nice keyboard shortcut for inserting HTML comments, however.
You can configure Kate to run external scripts, which is handy for power users. And the built-in support for CVS (the open-source change-management software, not the pharmacy) is a plus for those working on open-source collaboration. However, I don't want to write my own scripts or shortcuts to do simple things like add <b> tags. If that functionality is in the software, I couldn't find it again, and the documentation was a bit sparse. Kate looks like a nice piece of software for the specific functions it's aimed at, but it wasn't for me.
Kate ratings (on a scale of 1 to 10):
Ease of learning and use: 5
Look and feel: 6
Content editing (spell check, search/replace, etc.): 9
Simple HTML editing (bold, line breaks, ordered lists, etc.): 3
Customization (macro power, ease of creating): 3
Total: 26
All-in-one
This turned out to be a rather slender category after one of the apps I'd tested, EditPadPro for Linux, was discontinued. Still, one worthy entry remains.