L1A1 has a muzzle device that likes to go banana peel
L1A1 muzzle device is threaded in the opposite direction from all the metric barrels.
L1A1 has a gas port drilled at an angle (making it harder to modify)
L1A1 has different hammer/trigger/sear parts than metric.
L1A1 has different lower receiver than metric (can't use metric stocks)
L1A1 has a fatter bolt hold-open device (can't use L1A1 bolt hold-open device in metric)
L1A1 has a larger magazine release lever (mostly a matter of taste, but heavier)
L1A1 magazines only work in L1A1's unless permanently modified
L1A1 magazines much more expensive than metric
L1A1 magazines won't take metric floorplates, springs or followers
L1A1 buttstock has a buttplug that makes it harder to remove the stock
L1A1 buttstock has hard plastic or hard metal buttplate
L1A1 has blocky pistol grip with "popsicle stick" thing that can be troublesome
L1A1 is very difficult to get US made stocks or pistol grips
L1A1 has folding cocking handle that some folks don't like
L1A1 top covers won't fit metric - but metric will fit L1A1
I remain confused about whether L1A1 bolt parts interchange with metric.
I do know the extractors will not interchange.
You can exchange entire assembled bolts L1A1 to metric and vice-versa
All FAL bolt carriers interchange.
There are some other minor differences
Aftermarket scope-mount topcovers fit L1A1 or metric
Its all good.