Marriage

As long as one can't influence another's salary/work/promotion/hike etc it shouldn't matter(say different team).

If it does, then it's an issues

Other than, both husband and wife being in each other's face 24*7, there wouldn't be much issue. :)

It's a bit weird if one of you is a manager to the other. Otherwise, should be fine.

Nope. As long as you’ve made all necessary disclosures.