Halliday (2008: 49):
Some interpersonal meanings are highly generalised, like the enactment of dialogic rôles (speech function) … . With options in the way something is evaluated (“I approve / I disapprove”), or contended (“I agree / I disagree”), the borderline between grammar and lexis is shaded over; systems of appraisal, as described by Martin & White (2005), represent more delicate (more highly differentiated) options within the general region of evaluation.
On the appraisal system of attitude, Halliday (2008: 179) writes:
This is a grammatical system that is realised by a selection of lexical items. Each such item is uniquely identified as a set of intersecting grammatical features; eg complicated isappraisal: attitude: appreciation: composition: ( complexity : complex / polarity: negative …)
as well as other general grammatical features (e.g. as distinct from confusing, which is “effective”, as in it confuses me, complicated is “descriptive”). Note that I am interpreting the feature "undesirable" (the "snarl" member of the "purr/snarl" opposition) as negative in the environment of the interpersonal.