Bob is taller than Bill.

[Mr. Smith G.]

1.Bob is taller than Bill.
2.Sally is taller than Barbara is.
3.Bill studies harder than Bob.
4.Bill studies harder than Bob does.
5.I am as tall as Bob.
6.I am as tall as Bob is.

Are all of them OK?

 

[Barb_D]

Yes, all are okay, which shows you can omit the verb in 2, 4, and 6.
I suggest that 1, 3, and 5 are all more natural.

 

[Mr. Smith G.]

Buildings in the USA are taller than in Spain.
Buildings in the USA are taller than they are in Spain.

Are these both fine?

 

[MikeNewYork]

Yes.