Speed. Even basic levels of 'high speed' (ie. 180-200bpm) have always been a huge struggle of mine. I'd put the work in, and then get frustrated and lose my commitment to working on it due to no perceived gains.
Lately that's changed though. Witnessed Derek Roddy at Drumscene Live show in Sydney back in 2009, I think it was, and out of that got into a few specific bands that have made me addicted to fast metal and blast beats: Serpents Rise, Nile and Wintersun.
It wasn't until I finally forced myself to learn to play French Grip that I found I'd hit a turning point. My normal technique started feeling a lot better, more relaxed, and less awkward. I started doing things like George Kollias' 200 - 280bpm exercise (adapted to speeds I can play). Now I mix up doing 20 minute sets of continuous single strokes with the feet and hands playing simultaneously, and then working at the high speeds (200-210bpm at the moment) immediately afterwards. I make sure I'm correctly warmed up, smash out a couple of mid-tempo metal songs with lots of straight double kick, and on days when I'm having "good days" with regards to how I feel physically, I try and do as much fast music as I can, have a go at a couple of the slow songs with blast beats (they tend to be about 180bpm - 210bpm), and sometimes even make it to the end of the song without giving up!
I think the endurance work is what is generating the most obvious payoffs at the moment. I don't have the latent stamina to do it every day, but I'm working towards that. At the moment it's every 3rd day or so. Playing a 1 hour set is one thing, but playing continuous, non-stop singles even at only 155bpm is hard work when you're unaccustomed to it. The fact that I'm seeing an immediate increase in top speed even immediately after the end of the 20 minutes says quite a lot.
Oh, and I readjusted my slave pedal to match the master identically (turns out it was quite badly out), and now I'm smashing alternating double strokes at 200bpm after only about 2 weeks of working on doubles again. It's doing my head in. I've started working on heel-toe as well, just for the hell of it, but I'm definitely going to be putting some serious work into making those fast double strokes usable!
Sorry, I'm the sort of person who will talk for hours about drumming if you get me started, I will go away now