Adult vs kid's bicycle for a tall kid? My daughter is 11 years old. She is growing fast and for her age, she is quite tall (154cm). Her bicycle is too small for her now so I'd like to get her a new one.

The question is: should I look for a kid's bicycle for her or would it make more sense to get a normal bicycle; maybe something like a XS size? My guess is a frame size of 48 ~ 50 cm should be right for her. Do kid's bicycles have anything especial besides being maybe simpler when it comes to using gears, etc? Would an adult bicycle be too complicated for her?

Her old bicycle is an Islabike which is around 9 kg and comes with SRAM X4 Gripshift (32 tooth chainring with 12-32 wide range rear cassette). It was simple enough for her at age 7, when bought it.
You are correct that there isn't anything intrinsically different between bikes designed for preteens and adult bikes. Kids bikes tend to have lower end, inexpensive components and drivetrains with a single front chainring only.

A adult bike will come with multiple front chainrings and front derailleur and shifter, so your daughter will have to learn to use those as well as the rear gears, but that isn't particularly difficult. Less expensive mountain or hybrid bikes will come with a triple front crank, if you want to simplify gear shifting  to start with put the chain in the middle ring just shift the rear gears.

One concern would be the wheels may have quick releases instead of nuts on threaded axles, which might become unsecured with obvious bad consequences. The quick releases would have to be tightened enough so they couldn't be undone easily, and your daughter would have to know to not ride the bike if one or both come loose.