How much League of Legends cosplay was there at PAX Prime? A lot. For example, there's a person in that cat costume.
Lots of fans paid homage to Riot's online phenomenon by dressing up as LoL characters at Penny Arcade's annual fan gathering in Seattle. These pics come from documentary series/social network Cosplay Boom and all photos and stills were taken by Todd Kenreck and Meagan Sadler.) You can see previous round-ups of PAX Prime cosplay here, here and here.
LovelySkylark as Blade Queen Lissandra from League of Legends
Sky and Michelle Corbelly as Siris and Isa from Infinity Blade 3
Glen Macdonald as Leona from League of Legends
Kiara Boggs as Lux from League of Legends
Master Yi from League of Legends
Puppichu Cosplay as Nidalee from League of Legends
HeartLockett Cosplay as Tibbers from League of Legends
Trundle from League of Legends
Warhammer 40,000 soldier