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.
1) There's already other threads as pointed out.
2) There is no ability bloat for DKs imo. I use everything for every spec outside of Necrotic in PvE.
Using your own examples:
I could see the problem with multiple ways of applying diseases, but if you make Outbreak baseline and remove IT/PS then why bother having two diseases at all. Merge them into one? It also has no cost currently so a few things would have to change (cost, again), where we get out dispel from, and then what about Howling Blast. Why still have that apply Frost Fever? Then if they remove it, it would require you to use Pestilence, which is just annoying as Frost. Basically just messes too much up to bother.
Necrotic is, as you said, unique and is a big part of DK PvP. They could merge into with SS and make it be a DoT (and how would that work, Ignite-style?) on PvE targets but then that removes the choice between damage and stopping heals in PvP. People just have to learn to heal the absorb rather than heal damage after with Death Knights. If people can't understand that then...
Death and Decay is just too useful to be removed, for the same reason they let Pallies drop Consecration anywhere via a glyph. Pulling out stealthies who you watched run at you and stealth - putting it down in front of a patrol and then moving back - picking up adds on their way to you, someone else, or where you are about to go. Again the big part is the range. Blood Boil is pretty meh - and after just being the bat-kiter on Tortos HC (I did it as Frost after a few tries as Blood) - I can say it is definitely not enough for picking up adds and such.