The Whitecloaks were shown to be pretty popular among the common people during the early part of the series where quite a few people activly feared the Aes Sedai.

Secondly the Whitecloaks drew its members from every nation which give them a certain international clout. The only other group that could boast the same would be the Aes Sedai. They were also spread out which meant that they were probably better informed of what was going on that most other groups. Their numbers might be small but anyone who though about taking on a patrol had to know that there were a lot more Whitecloaks around and they could be called up if there was any trouble.

A lot of the nobles might have been wary of the Whitecloaks but as Lordling said before Rand & the Aiel and the Seanchan having 10,000 well trained soldiers in a standing army was considered quite respectable. While we have not seen much of the Whitecloaks in action I would expect that they get a lot more training that a typical armsman who may well have been pulling a plow before being recruited and given a few weeks training to know one end of a pike from the other.