very plain - this is "americanized" thai food. i liked lanna, until i explored thai food more and learned what authentic thai food tastes like. lanna's food is average, and it's "americanized" in that the spices and flavors have been tamed. i do not refer to the heat, rather i refer to the flavors. it is missing the aromatics, and the variety of scents and flavors authentic thai food has (or that even above average americanized thai food offers).
i've traveled to various parts of thailand. a dish with the same name may have many different twists on the concept. for example, interpretations of tom kha vary wildly...they all contain cocounut milk, kaffir, galangel, but some have tomatoes, others do not. but all -good- thai food should be aromatic, and one should be able to taste the separate flavors of sour, bitter, sweet, flowery, etc. these various aromas and flavors are missing at lanna thai. it all seems to run together.
thai chef is gone, thai siam has gone downhill. thai village has a pretty limited menu and is americanized too, but at least the dishes are aromatic, and the thai items they do offer are more flavorful. thai village has a narrower selection but if you are a "foodie" then it is a better choice than lanna. i hear there's a new thaiplaceat 30th and harvard that is closer to authentic.
update: recently i was told if you ask the chef to make 'authentic' or traditional thai food at lanna, that they will "un-americanize" it for you.