Defining "rock" through a checklist of features (4/4 rhythm, guitars, beats on 2 and 4, guitarists who trash bathrooms) is a fools' errand. For every possible item on that list, there are counterexamples, and usually you can even find counterexamples within the canon of rock classics.
Personally, I find it is better to think of Rock as a loose tradition, rather than a genre. Rock has existed for almost 3/4 of a century now, of course it's changed in that time. You'd run into the same problem of vague genre definitions if you tried to define how "classical music" sounds. Every step on the way from Blues Rock and Rock&Roll to the huge landscape of rock and metal genres we have today is small enough to appreciate how we got from here to there, but overall, it's like comparing an expertly crafted hole in the ground with a skyscraper. Of course, they do not seem the same - time has passed between the two.
A better way to define Rock, in my opinion, is along the lines of "I know it when I see it". There are certain elements and signifiers that are common in Rock, and when a couple of them come together in a song, that song probably is Rock. In the absence of these signifiers, it probably isn't. Some of those elements are: a drum kit, an electric bass, at least one guitar, maybe an organ or later a synthesizer, a vocal line which is prominent in the song and provides a lot of the song's melodic interest, a structure of tense verses and resolved choruses, aggressive timbres and dynamics, heavy beats, extensive guitar solos which focus on technical craftsmanship, vocal techniques that range from fairly standard singing to high-pitched screaming (cf. "Child in Time", Deep Purple) or low-pitched growling (too much Death Metal to list it all), and quite often some spirit of rebellion, be it against "the establishment, or be it against prevailing trends in music. This list is definitely incomplete, and certain subgenres may value some of those points more than others.
While Rock is often associated with being harsh, and with being simple music for idiots, both of these clichés are patently untrue. Rock can be a very gentle and emotional genre (a lot of Clapton's work would come to mind), and it can be very complex at the same time (cf. Dream Theater, or the aforementioned "Child in Time", or the entirety of Pink Floyd's "The Wall").
We also have to acknowledge that the influence of Rock goes far beyond Rock itself: While Lady Gaga certainly isn't a rock musician, and wouldn't claim to be, she lists Iron Maiden's Bruce Dickinson as one of her influences as a singer. No surprise there, seeing how vastly popular and completely inescapable Rock has been for the last 70 years.
I realize that this reads as "you're asking the wrong question", but the point is, there is no neat, simple answer to this question. People love simple answers, and there have been some, but sadly, none of them really get down to the heart of the matter.
tl;dr: What is Rock? It's complicated. Don't try and put all music into neat little boxes, because music isn't made to fit into neat little boxes.
