One explanation for the DL and CO similarity is that they both are dedicated to Boeing. CO's president is of course a former Boeing exec, and DL is just too 'conservative' to order a non-Boeing product.
Its also interesting that DL and CO come up periodically as possible merger partners. In fact, they have had high level discussions at various times over the years about this. I assume their fleet similarity is a big reason why they both are attracted to each other.
The only meaningful similarity I can think of between UA and BA is their importance in the 777 program. I think UA was the first domestic and BA the first international carrier to fly it, and of course they were heavily involved with Boeing in the design phase. Other than that UA and BA don't seem paritcularly similar to me.