Personally I think the two main difficulties are the diglossia and the vocabulary. I think the diglossia makes it hard to use the sort of fun-ish media-based 'acquisition' approach that many autodidacts are used to. It also means you need much more input to be able to properly differentiate the two codes and use them actively, which is difficult to get when you're starting at such a low level of comprehension (which is the case if you don't already speak a Semitic language or a language with a massive amount of Arabic loanwords).
I don't think that the issue with the vocabulary is that it's extensive or that there are lots of synonyms. Even if the dictionary is full of different words, I think in general every language will show a similar lexical/grammatical load when it comes to actual texts/speech. It's just that the vocabulary is very distant from European languages, and the foreign script also makes remembering new word roots more difficult at least at the beginning stages. Russian may be fairly lexically distant from English for example but it still has plenty of Greek, Latin and French loans, as well as some common Indo-European vocabulary and a somewhat more familiar syllable structure, that makes Russian simpler for speakers of Western European languages than Arabic. Already recognising some words goes a long way towards making input comprehensible enough so that you can learn from it, whereas if you recognise literally nothing you have to try much harder to pick anything up.
Honestly I don't think many languages of Asia, Africa or the Americas would be particularly easy for speakers of languages within the orbit of Standard Average European. Hell, even Basque and Finno-Ugric can be quite a challenge. It's just that Arabic has a lot of us complaining about it because it's big enough and visible enough that it draws quite a lot of interest, or at least more than Somali or Tamil.
That said, I think we should all just keep plugging along and do what we can given our level of motivation and need. Eventually we'll get there, it just takes more hours than we're used to from other languages.