The T in IObservable<T> is more powerful than you may realize.

Structural T

T carries data, its most obvious purpose.

T can be any type, such as Int32, String or MyAmazingObject.

T can be void, as in System.Reactive.Unit.

T can be anonymous, as in:  new { Value = "T", Awesome = true }

T can be a choice, as in Either<TLeft, TRight>, which also seems to fix so-called "glitches".

T can be a group of T, as in IObservable<IGroupedObservable<K,T>>; e.g., GroupBy.

T might be there or it might not, as in Maybe<T>.

Meta T

T carries notification.  Observables notify observers by pushing T.

T can be notification reified, as in Notification<T>.

T can be collection change notifications, as in CollectionNotification<T>.

T can be directed mutations, as in CollectionModification<T>.

Temporal T

T carries time.  Observables push T asynchronously (though sequentially-§4.2).  Even pushing synchronously takes time though.

T can be a point in time; implicitly, the time at which it's observed, as in Amb or CombineLatest.

T can be time reified, as in Timestamped<T>, TimeInterval<T>, Recorded<T> or simply DateTimeOffset.

Temporal Window T

T carries duration.  A window is a duration of time that has a start and maybe an end.

T can start a window, as in Join, GroupJoin, Window or Buffer.

T can end a window, as in Join, GroupJoin, Window(closingSelector), Buffer(closingSelector), etc.

T can be a window, as in IObservable<IObservable<T>>.

T can be the end of time, as in that which precedes it, interpreted relative to the time of subscription (e.g., Timer), relative to a previous T (e.g., Interval or Throttle), or relative to some out-of-band time (e.g., Sample).

Channel T

T can be a channel or callback.  Observers invoke T to affect an observable.

T can be a delegate, as in Action or Func<TResult>.

T can be a subject, as in IObservable<ISubject<T>>.  This establishes a full duplex, asynchronous communication channel.  The observable is a singleton and both ends subscribe to the subject; e.g., a chat application. (See also this blog post.)

Future T

T can be the promise of a future T, as in IObservable<Task<T>>; although, it's somewhat redundant since IObservable<T> is also a promise of a future T and it has a wider cardinality [0,∞].  Task's cardinality is strictly [0,1].

Conclusion

T carries data, notification, time and duration.  It can also have communication channel or promise semantics.  Best of all, T carries any additional semantics that you choose.

So the next time that you find yourself stuck trying to figure out which Rx operator is the right one to solve your particular problem, pause and give T the attention that it deserves.  You may just find that the answer to your problem lies within how you make use of T.

So, have I missed any categorically interesting uses for T?  How do you use T?