For me so far it is weather, followed up by radio. I was schooled in and worked as an electronics technician so it is not any of the technical aspects of actual electronics in radio, but the types of airspace, the different calls one has to make. That is all new to me and though I can do the alphabet, I'm struggling a little to what requests and demands on the radio.
But yes, weather so far also is the one I'm currently struggling with. I am getting some of the hang of the cloud types. The thing is often the sky seems to me to be filled with different types all together at times. It doesn't help reading it in a foreign language, but I do go to the english version from the FAA and it seems about the same really. Funny thing, I actually LIKE learning about weather...if I could just LEARN it. I mean I see the value not only in flying but also camping, boating, planning anything, etc. But at times it seems like cold/warm/dense/less dense/humidity high/low ALL always combine to make storms and clouds and rain...except when they make sunshine and daisies.
I keep reading long explanations about what happens when fronts meet (though I don't get why there is more than one, they meet or they don't but somehow it is "when a cold front meets a warm front" is different than when "A warm front meets a cold front", where one slides up on it, the other...oh hell, I'm stopping here.
My wife thinks I've hit bottom, as she sees I'm DVR'ing on the tv (I think it is Discovery channel) "Richard Hammonds' extreme weather" and I'm rooting each show for him to not talk extreme but just "normal" weather which actually he does. My first flight last month, afterwards my instructor also impressed the hell out of me (besides impressing me with the smooth landing in fairly good crosswind) when looking out the window in the clubs room after pointed to two cumulus clouds, one that was "pregnant" and might soon dump but when I asked about the other "no...it's afternoon, it won't have time to build up into anything". That's the kind of knowledge I want to have!