People spend years mastering their instruments to make the music and write original riffs and solos. The vocals are hard to do (not even talking about screaming but like Bruce Dickinson singing with power) and actually have deep and well thought out meanings. Compare that to pop and rap, which takes no time to learn an instrument. As someone who loves rap and metal and plays drums guitar and bass, metal wins in a landslide.
No other genre of music has you and 4 others playing 5 different meters at once. Meshuggah does this for example. It's hard enough just playing with only 2 meters at once, so when you got 2 or more you really have to communicate with your fellow musicians or you easily mess it all up.
Metal and rock music tend to have the most complexity and rhythm in terms of guitar and drum playing styles e.g. palm muting, tremolo picking. Thrash beats or blast beats on drums etc. The chords and progression of these instruments along with the vocals can be intimidating for first-timers who are getting into metal music but once these people discover more bands they like they'll see that the melodies and harmonies in the songs are breathtaking and would take them on a musical journey that would be within their hearts forever.
Metal is just some kind of perfection in my opinion, not only the skill, intensity, but also the determination and also those very intense aura and excitement. The music, as we know it, are absolutely unconventional, Therefore more skill is required. Also the most performance was almost always flawless, intense, and amazing.
If you're comparing Metal and Electronica, obviously, it takes way more skill to hold a guitar, coordinate left and right hand on strings, and demonstrate other techniques live, then to click a play button on a computer that plays music. If it's Jazz like the post to our right says, it takes just as much skill. There are different sounds and elements that make up these 2 different genres, but it doesn't make either on inferior to the other. Depending on how good the player is, and how long they've been playing. Personally though, Metal doesn't necessarily require more skill, but determination. Like what watchgal above said about screaming. It takes a long time for your vocal chords to be trained and worked out enough to not damage your voice.
Learning how to scream like they do in bands under metal sub genres, takes a lot of effort and work to do. It's not like you can just go ahead and scream along to a song without practicing how to get your voice to actually do it. You need to know what you're doing, because if you don't you could end up ripping up your vocal cords. That's just my opinion, though.
Jazz forces the musician to improvise a melody for a solo or a harmony. All there is a main melody and a beat, it's up to the musician to make it great from that point. Not only do you have to improvise it is a lot more sophisticated in terms of music theory which also makes it harder to play. Whereas rock and heavy metal are based on blues which is usually based on three simple chords.
Are you talking about progressive metal, or punk rock? Obviously playing the guitar, drums and bass is usually more skill dependent than modern pop music. However there are many genres such as EDM and Jazz which also take a lot of skill and time to learn how to do well. Jazz requires impressive knowledge of harmony, melody and improvisation. A rock song might only ask the guitar player to play 3 different power chords throughout the whole song.