not decorated in any way; with nothing added: The food is pretty plain (= prepared simply and without strong flavors), but there’s lots of it.
- a. : lacking flavor
e.g. This dish tastes a little flat
- Lacking flavor: mugs of insipid coffee
American Heritage dictionary
- Lacking taste, zest, or flavor; flat: vapid beer.
lacking taste or flavor or tang
having little or no flavour : These grapes are completely flavourless.
I would say "plain/flat" is most neutral, "vapid/insipid" less so, and "savorless/flavorless" least so.
some other options: zestless, lacking tang, dry, dull, unsavory, plastic, garden-variety, so-so, humdrum, cut-and-dried