In my experience as a teacher, some letters don’t get enough attention:
ل — The /l/ sound in Arabic is often realized as a light /l/ in positions where it’s realized as a dark /l/ in English (and other languages). This isn’t difficult to correct, because both sounds exist in English, but a learner can go a long time without being made aware of the difference.
ه — The /h/ sound is often more aspirated in Arabic than it is in English (and other languages).
ح — It’s often the case that a learner doesn’t even know that they don’t know how to produce the /ħ/ sound. I’ve had intermediate students who couldn’t even hear the difference between the ح and ه. The reason is that, as I’ve mentioned above, the ه is often more aspirated in Arabic, so it’s actually somewhere between the English ‘h’ and the ح. A learner might be convinced that they’re producing the /ħ/ sound when they’re in fact producing the more aspirated /h/ sound.