I've never spawned a Wither.
I've never fought Jean (that's the enderdragon's actual name) in survival mode.
...hang on, those are vanilla things.
I've never built a real sorting system. I've moved some items around in pipes or tubes on occasion, but never truly sorted. (I do have the makings of an AE system ready to install but I don't have the place to put them or the power to run it quite yet)
I've never run a boiler. Actually I made my first setup yesterday but I haven't quite got around to starting it yet. (Actually I want my AE system to supply wood to the boiler but I want the boiler to supply power to the AE system... and I need to finish another level of my base first since there's no room and the ceiling still isn't finished...)
I've never made or operated a nuclear reactor.
I've never made or used a turtle other than a mining turtle, and I've never used a program other than tunnel and excavate. Even though I'm a software engineer and am familiar with Lua, so I don't really have an excuse.
I've never found, made, or used iridium, platinum, or chrome. I just never got that far (and was kinda scared off until now). Yesterday I made my first massfab and made titanium, so this should change soon.
I've never made golems or any of the fancy TC wands, even though they look incredibly awesome, because I don't really have the patience for research, but mainly because I'm afraid of messing up the world with flux or wasting the vis, even though that's probably an irrational fear.
I've never made or used a router, or wrath lamp, even though they look pretty useful and awesome.
I've never made a mod, even though I figure I could.
I've never made a frame quarry, as epic as they seem, mainly because I don't have a sorting system, and I like mining.
I've never made a train or any of the more complex kinds of rails or a signalling system, even though I like trains a lot and have had some interest in model rail. I only just realised how odd this lack is.
I've never made Aerial Faith Plates... and I really ought to fix that very soon
Oh, and I've never made a XR handgun, though X3n0 is awesome and I've wanted to try his mod for ages and I've had it installed a few times.
I've never made liquid fuels other than BuildCraft's regular yellow refined oil fuel, or Forestry biomass, and I never used the biomass (mainly because that world deleted itself before I could get that far).
[edit] Oh yeah, I've never used any type of Forestry farm, or bred trees. And I haven't really bred bees. I did do a few runs of bees but if I got any new types I don't really remember.
And probably a lot of other stuff I've barely noticed is installed.