Why most smaller miniPCI equipped routerboards have 10/100 Ethernet port?
Doesn't it limit their throughput when used with N cards?

I haven't worried too much about it because the throughput is different than the Over the Air rate. On those boards they are most likely going to run out of CPU before they run out of 100 meg of ethernet.

Could you elaborate?
If 411 used as AP, i.e. no NAT/firewall/QoS why can't it handle normal N rate in close range?

yes... uses RB600 or RB800
In the near future they can put gigabit port in rb433xx I guess the proccessor is capable only the RGMII is gigabit

Citing from description of R52Hn:

High Performance (up to 300Mbps physical data rates and 200Mbps of actual user throughput)

I see people still reporting under 100mbps throughput regardless of the board - where's the bottleneck?