Well, rahmu, a router is basically an ethernet switch, a wifi transmitter, and a chipset. They are ubiquitous and can be found in every single wireless router you purchase nowadays. However, it's the specifications of those components that matter:
1) Ethernet switch: It should ideally be 1gigabit ethernet ports. With HD movie streaming, file transfer, backup,etc... network bandwidth higher than the 11MB/s (realistic) transfer speeds of 100mbit is required. When your internet connection is fast enough too (let's say over 40mbit),
internet access, coupled with any local network services being used, would make quite a large dent on the network bandwidth available. This is actually advised in here:
http://wiki.openwrt.org/doc/howto/bwmon
"Does your local network provide enough bandwidth?
You should make sure that when you do have a 50 Mbit/s line that your LAN has at least 100 Mbit/s. A Gigabit LAN would be recommend for bandwidths greater than 40 Mbit/s, because otherwise your LAN bandwidth might be consumed by local file transfers or other LAN services
being used by other clients connected to your router. Try not to make internet bandwidth tests from your wireless network and instead test your wireless network separately by transferring data from cable-connected clients (Ethernet or Fibre) to wireless clients."
Ideally, a gigabit port would offer 90+% of the theoretical speed. Meaning with a good ethernet switch, you should get above 900Mbit/s (or 112.5MB/s). For example, the Realtek RTL8111F ethernet switch has a 944Mbit/s throughput, which means 94% of theoretical rated speed of 1Gbit/s. (couldn't find the link now)
http://152.104.125.41/products/productsView.aspx?Langid=1&PFid=5&Level=5&Conn=4&ProdID=291
2) Wi-Fi transmitter: If the Wi-Fi transmitter is Atheros, then it would work better (more stability I guess, maybe better speeds) with Atheros wireless clients. If it is Broadcom, then it would work better with Broadcom wireless clients. Some OpenWRT builds may have problems with a
specific model for one of these brands. Some may or may not support additional features.
Detachable antennas are also a big plus for those wanting better range or directional range. More powerful omnidirectional antennas offer better range in all directions, while a directional antenna focuses the signal in a specific direction. These are useful for outdoor use, for example: directing the signal towards a swimming pool so that you can use your laptop / tablet while lying around.
Additionally, support for both 5GHz and 2.4GHz (dual-band) transmission may be beneficial. 5GHz may offer better speeds but lower range than 2.4GHz. 5GHz may also avoid interference, since the 2.4GHz band may be congested.
3) Chipset, CPU, RAM: The faster a router's CPU is, the more connections it can support. A low-end CPU may bottleneck applications which establish a huge number of connections (p2p, torrents). A big enough amount of RAM would allow multiple packages and services to run on the
router, increasing productivity (running an SIP server, VPN, network bandwidth consumption monitoring, etc... all services, of course, require RAM. A low amount of RAM would necessitate using a swap partition on a USB mass storage device connected to the router.)
4) Flash chip: In case one is thinking of replacing the stock firmware on his router, the router's flash chip should have enough capacity to store at least the custom firmware + a few packages. OpenWRT has a lite version and a full-fledged version, allowing its installation on
Other packages can probably be installed on a USB flash drive connected to the router, if the
custom firmware supports it (native support, or via packages).
5) USB port:
For storage: Connecting USB flash drives, hard drives, etc... and sharing them over the network.
For printers: Connecting a printer and sharing it over the network.
For backup, emergency internet: Connecting a 3G / 4G USB dongle, and allowing the whole network internet access. This is great for a backup plan, when the broadband internet goes down. Also, it allows the router to become a "3G router". You are then able to choose a router based
on other components and features, rather than searching for a "3G router" and getting stuck with its limitations.
I think these are some of the most important features one should look for in a router.