Speed,
desktop cpus are way faster (quad core vs dual core low power)
HDDs 2.5" Vs 3.5", no laptop drive can spin at 10K RPM yet there are tons of desktop drives that can
HDD redundancy, you cannot have raid 0/1/5/10/50 on a 15" laptop 17" will only do 0 or 1
multiple burners, most laptops cannot have multiple burners
HTPC, using a desktop comptuer as an HTPC is alot better than a laptops since most laptops do not have Optical out, HDMI/DVI,
gaming, ppl want a full sized KB, a FAST video card and a good mouse
the video cards in laptops are way slower than desktops since they are designed for mobility and low power consumption rather than raw processing power.
upgradability, you can only upgrade a laptop so much before its maxed out, with a desktop you can change any component easily, although you can change most components in laptops
reliability, desktop components should be more reliable since they dont move and get bumped around. also cooling is alot better, HDDs are more reliable if they are kept at 20C rather than 40C