As a fresh off IOE 727 FO, I'll add my 2 cents.
First...I thought learning to land the 727 was tough. I thought learning to fly as a committee (you talk, I'll fly, you get the flaps, I'll move the heading bug...that is if I'm on autopilot, otherwise you get the heading bug etc etc) was a new wrinkle. Here is what challenged me(and will likely continue to challenge me until I get more data points...)
CRM...FO talks on ground. Do I ask first before transmitting (no...not usually) or just make it happen. Remembering to take radios on landing roll AFTER captain takes jet back to taxi (not hard...just different). Watching 1 Captain always stay 1 VOR and inbound course ahead (I liked the tech) verses another stay on the same one with me (not wrong...) Adopting your style to match your captain's techniques and preferences. Even in a "standard" cockpit there is lots of room for different techniques.
Landing...first of all, airline guys make a BIG deal out of landing on centerline. He11...I usually land on my half in an Eagle, sometimes with another bro on my wing on the other side. I instinctively avoid the very center of a runway so my nose wheel won't bounce on lights or slide on paint and so my hook won't skip if I have to take a cable. Airline landing 101 is LAND ON CENTERLINE. Sounds easy, but almost always during roundout or flare 3500 plus hours of instinct and sight picture have me drifting 2-4 feet off centerline. I'll continue to work on that... The other issue is just getting the "picture" and "feel" of the various weights, CGs, and types of jets you fly. An F-15 with or without a tank fly just about the same in the pattern. With a myriad of variation of 727s here at FDX, plus different weights, CGs, and pattern altitudes, there is no "always" picture but rather a whole lot of variables you have to manage.
Autopilots. They seem to work great when you don't need them. They seem to s*ck when you are behind an arrival or departure and trying to get a free hand. I never had much GA time with autopilots, and the F-15 has a 2 toggle switches...attitude hold (and that is all is does) and altitude hold (whoopee!) Learning when and how to toggle between modes, and knowing when to lean on the autopilot (and when NOT to) involved some learning pain and knashing of Line check airman teeth.
Skinny parameters. What I mean is if I'm 3500 feet high on arrival or 5 miles from a crossing altitude restriction in an F15, I say "so what" and then climb/dive/add power/speedbrake/G it up as required to make things right. Other than considering the poor guy stuck on my wing (if applicable) there isn't much else to consider. If I'm 10 knots fast or 10 knots slow on an approach--who cares? I will fix it...now, on short final, or even in the flare if I want. I can fly 150-300 knots on arrival and 150-350 on departure and nobody cares. Now...find yourself way high on arrival or behind on a SID in an airliner and you are (usually) screwed. You can drop gear, use speedbrakes, zig or zag a bit but its all done by committee and nothing gets fixed fast. Additionally...too fast or slow on approach is an auto go-around, and while they are "free" you certainly don't want to make a habit or consistently needing multiple approaches to land. Conversely, if you don't like a pattern in the F15--take it around and do another closed pull-up...heck that is kind of fun anyway. The airspeed management is also an issue....speed up to 250 after clean up, then accel to 310 passing 10k, then slow below 250 on the way down, etc etc. You can do a lot more "seat of the pants" stuff in your military fighter. Perhaps with more time/seasoning I'll get those instincts in the 727 also, but for now it feels very mechanical and procedural.
And like everyone else has said...attitude is everything. Bottom line is I thought learning the new job was harder than I expected. However, I also know I've done challenging upgrades before and knew if I worked hard I'd learn. You will likely do likewise... In any case, good luck and enjoy!