Doesn't this seem familiar? That's because it is. Rock Band faced the same problem after it launched.
[UPDATE: Activision admits to drum set issues]
A number of users are claiming their shiny new Guitar Hero World Tour instruments are not working correctly. MaxConsole reports that an IGN Forum member claims, "from what I've heard 95% of people have their yellow cymbals broken or red pad not registering." That seems like an unrealistically high number, but there's no official statistics about the problem just yet.
The reported problems all seem to relate to the drum set -- particularly the red pad and the cymbals. Some players are noting that they have to hit these especially hard, and some say that certain colors aren't functioning at all. On IGN's forums, user kingallanon says, "I got the game and my yellow cymbal isn't working right and my red pad seems like it might die any time..." Poster evil ways at NEOgaf articulately asked, "Is it just me or do the crash and hi hat cymbals have to be hit like a fucking bitch in order for them to work[?] The rest of the pads are fine, but all the notes I'm missing are from those 2, especially if there's a lot of repetitions."
Red Octane and Activision have set up Support answers titled "All or part of my drum set is not working" for Playstation 3, Xbox 360, Wii, and Playstation 2. It instructs you to check all the dumb things like make sure the controller is plugged in and that it has batteries, and if you've done those, it directs you to the RedOctane warranty page.
While obviously it's impossible to prevent quality control issues like this, you think that Activision and RedOctane would work extra hard to make sure this didn't happen after the Rock Band debacle. Clearly, plastic intstruments are not the most reliable form of electronics, but if this is a widespread problem it's hard to see how it could pass the companies' testing.
We'll keep you updated if anything further develops.