Well, let me start this by saying that DK's dont have as bad ability bloat like hunters for example, but surely there are some spells that feel completely removable.
I myself play only PvP at approx 2.1k Rating, so my points are mostly from a PvPer stand point.
Outbreak: Well, when cata came, two of the three new abilities (Necrotic Strike, Outbreak) seemed like abilities which were made only because they had to make some new spells. Outbreak doesnt really add anything to gameplay, and honestly, removing it would be the first step to make applying diseases and keeping them up fun again. From just Icy touch and Plague strike, DKäs just have too many abilities applying diseases nowdays.
Necrotic Strike: This is an very unique spell, considering its meant for pure damage, yet it has only use mainly in PvP, now this is what makes us DK's have a bit too high damage in PvP, since normally damage is balanced around PvE damage, NS creates imbalance with DK's having a enhanced strike just for PvP.
Death n Decay: While i love this ability because of WC3, this seems really lackluster and strange ability.. It doesnt have any synergy with any of three DK specs, and just feels that it is in a really weird spot.. However, i'd like to see this ability in a new form or something, but i think we dont need this ability as long as we have blood boil, which is almost the same ability.
I see many people complaining about Dark Simulacrum, but i would cry if it were to be removed, as its the only spell that makes DK's somewhat hard to play properly in PvP.
Share your own thoughts and opinions if you feel like, and feel free to comment on mine.