Answer:
Bad things:
Power - a good LAN is required to be on all the times.
Security - each computer and device become another point of entry for undesirables.
Upkeep - when things go wrong or the software gets updated.
Frustration if having a problem setting up.
A lot of times a network shares one Internet connection - if all computers running at once, can reduce speed for each.
Area covered is limited.
Good things:
Easy to share devices (printers, scanners, external drives)
Easy to share data (homework, pictures..)
Build a complex enough one and you may get enough experience to get a new job.
Cost of LAN Setup is low.