Mostly, Gregtech.
Polarizes the community a lot. Some people love it, some people hate it. It certainly makes you get involved a lot more, and what it takes, it eventually gives back, but it's all at the cost of time, and learning how to work the dozens different recipes for the Industrial Grinde/Electrolyzer.
Most people, when not running Gregtech, tend to skim a lot over IC2 because BC/TE/RC is a whole lot easier and more rewarding than IC2 is. In fact, the vast majority of people end up only investing in the IC2 tech tree to get to Quantum Armor, and eventually to GraviSuit if it's installed. If neither of those is an option (like many servers), IC2 becomes very much restricted to Jetpacks & Drills. Thermal Expansion gave ore duplication to Buildcraft, making Macerators more or less obsolete, since using the Induction Smelter you can get Rich Slag from smelting Ores with Sand, which makes for a perfect match for when you want to smelt those very valuable dusts, like shiny metal ingots.
Gregtech gives you a reason to invest in IC2. It gives you the ability to get a lot of resources from the electrolyzer/grinder, at the cost of a crapton of EU, which you'll need a lot of infrastructure for. Having problems with Tin? No problem, centrifuge some Lava Cells. If you craft your cells out of Xycraft's Aluminum, you're even basically "transmuting" Aluminum into Tin, which is much more useful. Of course, you'll need 250k EU, and the process takes 2500 seconds, but hey, watcha gonna do? You installed Gregtech, man. That's your problem now.
Finally, DW20 will be getting a lot of mods that will not be added to Ultimate. Direwolf reviews a lot of mods. And i do mean A LOT. Sometimes, he finds a really cool and interesting mod that he wants to use in his LP, and then, bam, he's going to add it to his pack, so that his youtube viewers can get the mod and play his downloaded map. Not common, but he has added mods in the middle of LP's before, especially since they tend to run for at least 2 or 3 months.
Ultimate doesn't work quite like that. Process is a little more complicated, the staff that oversees the mod has to test the mod, test for compatibility, see if the community is interested in the mod or not, etc... Right now, there's a lot of overlap, primarily because DW20 and the people at FTB both play in Forgecraft and are in contact with a lot of the same mods, so it's to be expected. We will definitely be seeing a split between Ultimate and DW20 as time passes, as the focus between them is very, very different, and i doubt Direwolf himself will want his mod to be identical to the FTB Ultimate pack, as that would be quite boring for his viewers to see, and he's constantly trying to surprise his audience with new and interesting stuff.
