Asuswrt-merlin provides the following changes over the original firmware:

64KB NVRAM
WakeOnLan web interface (with user-entered preset targets)
JFFS persistent partition
User scripts executed at init, services startup, WAN up, firewall up and shutdown.
SSHD (through dropbear)
HTTPS support
OUI (MAC address) lookup if you click on a MAC on the Client list (ported from DD-WRT)
Optionally turn the WPS button into a radio on/off switch
Saving your traffic history to disk (USB or JFFS)
Displaying monthly traffic history
Cron jobs
Monitor your router's temperature (under Administration -> Performance Tuning)
Display active/tracked network connections
Allows tweaking TCP/UDP connection tracking timeouts
Various bugfixes: crash issues related to VPN, etc...
layer7 and cifs kernel modules added
Optional user-settings for the WAN DHCP client (required by some ISPs)
Description field added to DHCP reservation entries
Dual WAN support (both failover and load balancing supported)
Disk spindown after user-configurable inactivity timeout
System info summary page
Wireless client IP and hostname on the Wireless Log page