Troi was in danger of going in S1, there are three episodes in a row where she doesn't make an appearance, so had Yar stayed they might've killed off Troi or bumped her down to just a recurring character.
Wesley should've left with his mother, that would've saved us from all the awful child-genius moments.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)