Yak Meat is Beef! But different...... Yak is a bovine, so from a technical standpoint yak meat is "Beef". As with any meat, flavor is dependent on a number of factors. Breed, genetics within a breed, diet, age, environmental conditions,and the process used to "finish" the animal all play an important role in the end result. Yak meat is fairly rare at this point and each ranch or producer provides a unique product.
At the top of the "what does yak meat taste like" question is: "Are you tasting 100% yak or a cross with a domestic cattle breed". Yaks are crossed with domestic cattle for various reasons including hybrid vigor, growth characteristics, hardiness, etc. Crosses will produce very different results as you are introducing genetics from any of dozens of breeds of domestic cattle.
Within domestic yaks, there are varying characteristics as well. Full-blood yaks are typically less marbled than production beef cattle with most of their fat being carried on the outside of the carcass. This produces a lean product that has a jump on even grass-fed beef.
Finishing may have as much to do with flavor as anything. There are various methods and philosophies. Grain finishing: corn, wheat screenings, various rations, all produce different results. Grass finishing has just as many variables.
Add in age of the animal, environmental factors including stress, the butchering and aging process, etc. and you are dealing as much with art as you are science. Consistency in the flavor of the end product is elusive.
Oh yeah the someone has to cook it!
The great thing about yak meat is that you are starting with a naturally lean, high protein, meat with a great balance of fatty acids that is tender and delicious.
Any good yak meat discussion should end with a little cheese, "Once You Go Yak, You Will Never Go Back!"