I haven't played in a few weeks and I'm not at home so I'll try and go from memory as to which addons I use.
I've actually cut down quite a bit with my use of "UI changing" addons so it's now looking more like a "stock UI" than anything else. I tend to use many addons that I consider to be of "comfort-of-life".
I've included the name of the addon, the link to the URL where I downloaded them from, and a brief reasoning as to why I use them.
Here are the addons that are a "must" for me:
DeathKnight.Info Runeshttp://wow.curse.com/downloads/wow-addons/details/dkirunes.aspxI only use the Disease bars so I can track my diseases on the target. Another nice feature I find is the fact that it tracks pestilence as well.
ClassTimer http://wow.curse.com/downloads/wow-addons/details/classtimer.aspxI use this addon to track other things like Horn of Winter, Bone Shield. It can be configured for just about anything.
Recounthttp://wow.curse.com/downloads/wow-addons/details/recount.aspxExcellent addon to track where your damage is coming from and perhaps help you to improve upon maximizing your dps.
OmniCChttp://wow.curse.com/downloads/wow-addons/details/omni-cc.aspxA nice little addon that puts a countdown timer in your spellbox on your action bar to let you know when it's available again for use.
Gridhttp://wow.curse.com/downloads/wow-addons/details/grid.aspxAddon that replaces your normal raid frames. I like it because it's rather compact and customizable.
Deadly Boss Modshttp://wow.curse.com/downloads/wow-addons/details/deadly-boss-mods.aspxI like to use this because particular one because it's also usable in battlegrounds.
Proculashttp://wow.curse.com/downloads/wow-addons/details/proculas.aspxThis is an addon I've only became aware of about a month ago. It allows me to see what has proc'd so I can maximize my use of gargoyle. It's very customizable and I enjoy it tremendously.
Now here are the addons that I use to make my gaming experience a little easier but can live without:
EquipComparehttp://www.wowinterface.com/downloads/fileinfo.php?id=4392Basically the title says it all. You can compare an item that you don't have equipped with the one you currently have equipped from the same slot.
Postalhttp://wow.curse.com/downloads/wow-addons/details/postal.aspxThis is an addon that's good it you have to mass-mail items or receive alot of mail.
Auctinatorhttp://wow.curse.com/downloads/wow-addons/details/auctionator.aspxI use to use auctioneer but have switched to this one simply because I think it uses less memory and it does what I need it to do. (Basically mass auction items)
Cellularhttp://wow.curse.com/downloads/wow-addons/details/cellular.aspxI like this because my incoming whispers pop up in another messenger-type window.
Addon Control Panelhttp://wow.curse.com/downloads/wow-addons/details/acp.aspxI've saved the best for last. This addon is one of my favourites as I can pick and choose which addon I want to use while in game and then just reload my UI instead of having to log out, check/uncheck the addons and log back in. Makes my gaming experience so much better and convenient. My hat goes off to the author of this addon.
So those are the basic addons I use on a daily basis. Depending on what I'm doing: raiding, bgs, questing, will determine how many and which ones I'm using at any given time.
With respect to Rune Strike. I actually use a macro for all my strikes and have Rune Strike incorporated into it. That way...it is always being used when it's available.
ie:
/startattack
#showtooltip Death Strike
/cast Rune Strike
/cast Death Strike
You can find more at this URL:
http://www.wow-pro.com/class_guides/useful_death_knight_macrosHope this information is useful.