* Positive: eager, ardent, zealous, passionate, fervent, spirited, keen, excited, motivated, fired up
* Neutral: interested, lively, animated, engaged, energetic, vigorous
The antonym for "enthusiastic" is apathetic.
Other words that could be considered antonyms include:
* Negative: indifferent, unenthusiastic, uninspired, disinterested, reluctant, bored, listless, lethargic