I would never have called Motorhead music "complicated". I think the main reason Motorhead aren't as big as AC/DC is because of Lemmy's vocals.

However, you do have a valid point. It's exactly why so many people bought Metallica's Black Album, but couldn't get into the much better, classic ...And Justice For All album. Way too complex for most people.

Complicated? God...
Motörhead songs are pretty simple (from a standpoint of music-theory). AC/DC follows more the traditional blues schemes. Bass lines are simpler than Motörhead, but we all know Lemmy takes over the rhythm part.
Guitar stuff is very low level in Motörhead songs. AC/DC guitar parts are more, sometimes much more sophisticated.
However, Lemmys lyrics are very different from the (in fact) simple AC/DC stuff. But lyrics are totally unimportant, most people don't understand them, anyway.

The most important part (and I remember that very well form the 1970s an '80s) is that AC/DC had many "party songs" you could perfectly bawl along with. This of course boosted record sales...

I would say that lyrics are important to the direct or "Target" audience, after all if you write a very heavy song the lyrics have to match the mood or fans would sit there going "What the fuck is this?". Lyrics play as big a part as the guitar work in metal and rock, but sometimes they don't play as big a part as the guitar or rhythm work.

Sure mate, but don't forget that not the whole world speaks English

Almost nobody in Europe understands English lyrics (I'm talking about hearing them in a song, not reading them). Maybe the chorus, but not much more.

Many (non-English) people will disagree to that, but when you ask them what a song is about they will mostly look offended or embarrassed at you...

I see what you mean, and not every country will understand lyrics in a foreign language because of possible deep meanings past face value, but they still play a vital role. The vocalist and the lyrics they bring forward together form a key two headed machine to creatng a bands image, and the perception in the eyes of their audience be they commited English or even Japanese fans. I mean imagine how different Motorhead would be if Lemmy had (Horror of horrors) been thrown out and replaced, and (As a personal example based on my own tastes) how different Judas Priest sounded when Rob Halford wasn't there.

Lyrics are what backs up those great riffs that defined metal and rock n' roll, and foreign fans will learn the meanings or concepts of songs if they are so deeply involved with that certain band they love.

Okay, fully agree to that. But then lets call it "voice", not "lyrics".
The singer is part of the overall sound experience. Changing the singer usually does not work. Who, for example, could have replaced Freddy Mercury in Queen? Or Dio in Heaven&Hell? It's a signature sound, unique.

Motörhead is maybe the best example, at least for contemporary bands. Lemmys voice is one in a billion! The band would be nothing without him.

However, sometimes it works. AC/DC is an example. Many say (including me) that AC/DC stopped to exist when Bon died, but nobody can deny the success of "AC/DC V2.0" with Brian.

Yes Iron Fist, you are truly right. I`m from Finland and don`t alwayes understand lyrics when i hear them. But fortunately Motörhead albulm`s are usually included with lyrick`s atleast nowdays. I can figure them out after reading with dictiorary. My point of view is that the hole package is important. Drumming, guitars, singing and atmosphere of the song.

Funny story from early days: `i heard STAY CLEAN my first time early 80`s, was then teenager. I thought that what great song but why this man have to stay so clean, don`t he take never ever any shower`s after he get`s dirty. Until i undertood the song was about the drug`s. Atleast year or two i just thought that the man was dirty bastard.

Awesome! "Burn In Hell" is the best song they've ever written, and that comes from a man who thinks Rob is a true metal God.

To "snaggletooth" and "iron fist":

Yes Yes I see, but that what "snaggletooth" said about lyrics are important. Yes, Well thats what I thought when I said more complicated than ACDC. How much Motorhead is complicated is another story, I said it is more complicated than ACDC. Lyrics are important, on lyrics you can see way of thinkin of the band and how deep they are in music, you can see how they see this world, people etc... On lyrics you can see if you want to put that way how smart is someone. On lyrics you can see in Motorhead way of thinkin, Motorhead way of thinkin and music is art not only music for fun. Someone here said that many songs from ACDC are made "for party's", thats what Im trying to say. Songs: Orgasmatron, Im the sword, dont need religion, 1916, and many others are to me much more deep than ACDC stuff. ACDC is simple rnr for fun more or less, Motorhead is much more intellectual. Than you compare playing instruments ACDC vs Motorhead. Come on, Mikkey Dee is doctor for Phil Rudd. Phil Rudd plays more or less one rhythm on drums all the time, Lemmy plays bass much more than Cliff Williams and Angus great solos, I dont see that he have much more better solos than Phil, and Angus always plays solos on the same way more or less. So, Motorhead in intellectual way (lyrics) and musical way is much more progressive and if you want better than ACDC.

Mikkey Dee plays the same mechanical beat on every song, just like Phil Rudd, only faster. Philthy mixed things up a lot more.
Phil Campbell and Angus come from the same school of bluesy solos. Whatever Phil can play I'd imagine Angus could probably play better. Motorhead, for me, have never been about great guitar solos - every single one of the guitarists in Motorhead have relied on the cocked wah wah pedal as a crutch.