I was at karaoke and I thought “one” by U2 might be a nice easy song to sing. Wrong. The words aren’t the hard part, its trying to mimic the feeling in the song.
I can sing the hell out of that song at karaoke – but then, I’m a chick. 
I’ve performed that song (with a band, not at karaoke): what’s deceptive is that the part you’re talking about is actually neither particularly high-pitched nor screechy, it’s just loud. And, therefore, not very difficult. 
I will say that there’s something “deceptive” about Ozzy’s voice, in that he doesn’t sound like he’s singing as high as he actually is. So you think, “that sounds easy”, and then you try it and suddenly find you can’t hit the notes. Sammy Hagar, same thing.
If one is trying to faithfully reproduce the original, then “Johnny B. Goode” belongs on this list, since most people don’t realize that the last chorus is rhythmically different than the preceding two, or if they do realize it they are incapable of reproducing it. In the first two choruses, the first, “Go!” falls on the downbeat, and the “Go, Johnny, go!” begins on the upbeat at the end of the measure. The last chorus has the first “Go!” of each phrase begin on the upbeat before three and leading directly into the rest of the phrase, except for the final, “Johnny B. Goode”, which reverts to the original rhythm. (I probably screwed up the explanation a bit; if someone more musically inclined wants to explain it better, please feel free).
It’s my go-to song for live band karaoke, and the first time I sang it I got a raised eyebrow and an appreciative nod from the lead guitarist when I sang the final chorus a la Chuck Berry. I’d like to think I didn’t completely cock up the rest of the song either.
“All I want” by Toad the Wet Sprocket is deceptively hard to sing…in fact it was so hard to get right in the studio that the band almost left it off the album.
For me, most of Boz Scaggs’ hits are deceptively difficult to nail. If you go to a karaoke bar that shows you automatic feedback of pitch you can see just how much you miss by. I’m always slightly over or slightly under the changes.
I used to sing the backup female vocals for Pink Floyd’s The Great Gig in the Sky for my band, and got accused of lip synching more than once. That cover took us almost 3 months to perfect.
I find trying to sing along to Rammsteins Stirb Nicht Vor Mir hard, the make voice and the female voice are just different enough to switch between very much like Joan Baez and Jeff Shurtleff in Drug Store Truck Driving Man. [It doesn’t help that she and I have different base keys to our voices.]
Oh, and the other one was “Oh Yoko.” Easy to sing sitting on the couch (even in C-sharp), hard to sell the falsetto into a mike over a full band and (a) not sound goofy and (b) not pass out.
It’s only marginally pop music, I guess, and probably doesn’t get karaoke’d much, but I find it really hard to sing Nick Drake. I’m not sure why. He’s a guy with a low voice who seems to hang at the top of his range a lot, but there’s something about his melodies that just sound wrong when sung by somebody else.
I’d also add Jack White to the list of singers whose work is generally hard to reproduce. I don’t know how many utterly dreadful renditions of “Seven Nation Army” I’ve had to endure. I heard one last night which was actually pretty decent. I was astonished.
I don’t find it too bad, when dropped by one half-step. It’s just a little tricky to get the timing on the transitions between verses - “It’s been too long since we took the time” et al.
I’ve also been using a somewhat unconventional trick for “Imagine” – sing it to the tune of “Folsom Prison Blues”. They fit together almost perfectly.
Fortunately it’s a little too ironic for people to lynch me over a song about love and pacifism.
My downfall at karaoke was Toto’s “Africa”. It was that change in pitch for “Gonna take a lot to drag me away from you” that got me Every Single Time. I can do high, but that sudden shift is apparently beyond me.
If we’re including musicals, I know from experience that a lot of Sondheim is notoriously hard to perform . . . like Amy’s part in (Not) Getting Married Today. The amazing soloist is Beth Howland. Nobody ever performed this song better.
‘Music’ John Miles
Couple of key changes here and there, but its the way it keeps going up and steadily up, and with some power in the voice.You can probably sing quite a lot of it, but you’ll find some bits quite difficult.
That whole Kilimanjaro/Olympus/Serengeti line strikes me as very difficult.
To sing, or to make sense of?
Got one: (disclaimer: not a singer, but I play this on Hammond a la JOS or McGriff). “What’s Goin On.” Not only can I barely remember the whole form, but getting through all the horn hits and timing, and voicing is tough, for such a popular tune that everyone knows.