General lack of enthusiasm:
* Apathetic: Showing or feeling no interest, enthusiasm, or concern.
* Indifferent: Having no particular interest or sympathy; unconcerned.
* Uninterested: Not interested or enthusiastic.
* Listless: Lacking in energy and enthusiasm.
* Lackadaisical: Lacking enthusiasm and effort; careless.
More specific meanings:
* Disenchanted: Disillusioned; no longer believing in something.
* Dispirited: Having lost enthusiasm and hope.
* Demotivated: Lacking motivation or enthusiasm.
* Passionless: Lacking strong emotion or enthusiasm.
* Tepid: Showing little enthusiasm or interest.
The best word to use will depend on the specific context and the level of intensity you want to convey.