$x = '/home/public_html/wp-content/plugins/myplugin/myplugin.php';

//dirname($x) == '/home/public_html/wp-content/plugins/myplugin'
//dirname(dirname($x)) == '/home/public_html/wp-content/plugins'

Regarding trailing slashes, it’s a matter of preference, but a really good idea to be consistent within a particular project.

Personally I prefer no-slash, for two reasons:
1) It’s easier to add a slash when you need it than to take one off when you don’t.
2) “$x/$y/$z/file.php” looks more like a path than “$x$y$zfile.php”

Yes, trailing slashes inconsistency is sometimes annoying too. Anyway, I do not understand why not to *always* add trailing slash. Do you know any motivation for that?

RE constant names, I have no problem with the ones they’re using. They’re consistent, which is nice. My pet peeve is that ABSPATH has the slash at the end, but none of the other path constants do. ABSPATH is so deeply embedded it would be impossible to change at this point, short of just replacing it outright.

Before all other reasons you mentioned, this could create some new problems – like if another plugin would need the constant (then it would get it with unexpected value).

I think plugin coders (mostly) know, that when they need to store some plugin-specific data, they should use variables or constant of specifically prefixed names, or store them inside the own class.

And I also think that the names of new constants weren’t created too well. Instead of WP_PLUGIN_DIR and WP_PLUGIN_URL it would be IMHO more correct to make WP_PLUGINS_DIR_PATH and WP_PLUGINS_DIR_URL. (I’m interested in other’s opionions…)