Past me would be most surprised about my political orientations, I suspect. I was always left-ish, but younger me didn't really even have an idea of anticapitalism as an actual ideology. "Slightly more left than Obama" was simply as far left as politics existed, as far as I knew. Kind of low hanging fruit, but it's true; what can I say? They'd also be surprised (and disapprove) of how much music I listen to, and what genre. When I was younger I basically only enjoyed Celtic and New Age nature music (and enjoyed both only quietly). These does I have a higher tolerance for loud sounds, which probably hasn't been great for my hearing, and have an expanded genre range.
They would be shocked that I even considered touching Twitter, and probably pretty surprised at my current relationship with the housecats, which is more or less non-existent. For whatever reasons these ones don't enjoy cuddling with me.
That I'd gotten worse at long-form writing would disappoint them. They never would have expected that I'd grow so used to IRC roleplay that my other writing skills would atrophy. Oh, and they'd be surprised that I don't do glassworking anymore.
"If you are worthy of his affection, a cat will be your friend but never your slave. He keeps his free will though he loves, and will not do for you what he thinks unreasonable; but if he once gives himself to you, it is with absolute confidence and fidelity of affection." -Theophile Gautier