Neutral:
* Uninteresting
* Dull
* Monotonous
* Tedious
* Lackluster
* Uninspiring
* Bland
* Tiresome
* Repetitive
* Uneventful
Negative:
* Mind-numbing
* Droning
* Soporific
* Snooze-inducing
* Humdrum
* Stale
* Lacking in excitement
* Depressing
* Wearisome
* Unengaging
Descriptive:
* Predictable
* Clichéd
* Overdone
* Stagnant
* Lacking in originality
* Routine
* Conventional
Informal:
* Lame
* Blah
* Yawn-inducing
* A drag
* A snoozefest
* A bore
More specific:
* Uninformative (for lectures, presentations, etc.)
* Unentertaining (for movies, shows, etc.)
* Uninspiring (for art, music, etc.)
* Unchallenging (for games, puzzles, etc.)
The best word to use will depend on the specific context and the feeling you want to convey.