The way David describes it is that it does sound like arabic to someone who dosen't speak arabic since we are not really that familiar with hearing arabic so when we hear the velar fricative [x ] and the uvular plosive [q] it immediately reminds us of arabic.
Ya, the verb classes are intersting. Though it will probably be really hard to keep track of it and learn it all. There was a quote in an interview with the show creators Benioff and Weiss where they said that "I’ve been told Klingon was hard to learn, so we asked that it be easier to learn than Klingon." This is actually not true. David says that they never asked that. In fact Dothraki is much harder to learn than Klingon. What they are probably referring to is how easy it is to pronounce. It is at least easier to pronounce than Klingon is and that's really what matters for the show anyway since the actors won't be learning the language per se but only learning to pronounce it.
I think the doubled consonants also contribute to it sounding Arabic-y. And the rhythm.
Klingon is not that much more difficult to pronounce although the initial and final glottal stops on ’U’ (the opera) are a bit tricky. I'm sure that in the big picture of things, English is MUCH more difficult to pronounce than Dothraki, but not for
me because it's my native language.
I agree that Dothraki is very very difficult to learn (grammatically) and clearly David has made it that way (at least partially) for his own enjoyment.
He's doing it "because he can". It's certainly his prerogative.
It continues to remind me of Classical Latin from a learnability perspective.