I'd say that English is easier to learn than most languages (precisely because of that ubiquity), but harder to master. The analogy to PHP holds: PHP is everywhere (a lot of websites run on it), learning material is easy to come by, and it's widely understood (most commodity web servers will just take your .php files and run them without any trouble on your part), but it will take years until you stop tripping over its quirks.
Hardest part is what the author has mentioned: vocabulary. Those french words drive me crazy when I tried to memorize them for the sake of GRE. Very little pattern, and the pronunciation is also different, some of them I don't even know how to start, e.g. cliché.