They make incredibly polished games. (Even if you don't like the gameplay, you can't deny how much effort normally goes into their titles.)
They release free, significant, updates for their games for free (300 TF2 updates, Portal 2 map editor, L4D updates)
They don't actively try to fuck over their customers (Steam sales, Dota 2 having a F2P model that isn't a massive grindfest)
They provide things to service the community ala Steam Workshop (where people have been paid thousands of dollars for making hats) and Source Filmmaker (releasing the same tools they use to make their promos for free)
They actively listen to their community and release updates to appease to them. (As an example: Dota 2's in game voice chat was shit for the longest time, so they implemented a new codec that significantly improved the voice quality)
You don't have to like Valve games, but you have to admit they are one of the few bro devs in the industry.