Having spent upwards of ten years on 737's with FMC's, and now having spent a bit of time on the 777, I'm honestly baffled at some of the decisions Boeing made on the 777 FMC - to me, and I say this knowing it could just be my lack of knowledge, but a lot of it seems like a significant step backwards? Why do I (for example) have to input a speed AND altitude constraint at a point if all I wanted was a speed constraint? For example - in the 737, at Waypoint X, I could input 250/ if I wanted an FMC commanded speed in VNAV of 250 KIAS. The FMC would then show the predicted altitude at that point (let's say I'm in a descent) in small numerals. In the 777 I am forced to enter an at, 'A' or below, 'B' hard limit, which sometimes involves a bit of guesswork/predictions that the computer is supposed to be making for me (and is, in the background!) I also can no longer see the predicated altitude at that point assuming I remained in VNAV PATH on the profile.
If memory serves me right, I could also enter a Mach number in the 737 FMC as '0.78' or '.78' (the zero could be omitted, or not) whereas in the 777 the zero MUST be omitted (but NOT in our pre formatted ACARS messages, where the zero MUST be entered because we hate standardising), maybe my memory is playing tricks on me again though.
There are a few other points that baffle me as to why they are the way they are, but.....well, it just is what it is.