Deathwatch

Just a few - they are no challenge. UNLESS there is a group of them, then they can take down a lvl 60. 5 lvl 35s can take down a lvl 60 player, especially if that lvl 60 player isn't carefull, which they generally aren't.

But of course, there will be more sadistic people than me - and it will happen that people go after low levels, but it will be a rather rare occassion.

As for leveling together - it won't work. Some people will play more than others, and the first high-level character will become their main. However, eventually everybody will be lvl 60, and then you have high level content to deal with, such as the high level instances and raids.