Real World Motives Trump Game World Motives
Presumably, there is something to motivate the users or else they wouldn’t get involved in the first place. If not, then you’re being unfair to the users and you should provide some extrinsic reward, such as paying them. However, it sounds like you’re relying on volunteers motivated to do something for a worthwhile ultimate goal. If that’s the case, then understanding the impact of their work isn’t a barrier to engagement –it’s the key to engagement. You should periodically or continuously remind users how much they’re making a difference.
For example, progress stats (which, as other answers attest, is generally a good idea) should be expressed in real-world terms. Instead of just saying “1536 words corrected in scanned documents,” also say “on average enough corrections to spur 3 new historical discoveries,” or whatever the real-world impact is. Provide real-world feedback, such as a homepage news item of what was achieved (e.g., “Thanks to your corrections, Dr. Jones of Indiana U used our documents to show that the Incas were vulnerable to conquest by Pizarro due to political instability in the northern Andes” -link to publication).
Types of Boredom
Beyond the extrinsic and intrinsic motivators of the task itself, you can do a lot by addressing the nature of the boredom. When you say your task is boring, what do you mean exactly?
Is it tedious? That is, is there a lot of pointless work? While the overall task may have a point your users believe in, maybe there are some steps that are really unnecessary. Make sure you UI is clear of all “excise” work that can be streamlined. This can be as simple as pre-positioning the cursor in the text blank with each page load so users don’t have to click there themselves.
Is it monotonous? That is, is it the same thing over and over again? If so, see if you can support more automation. For example, allow the user to multi-select objects and perform the same operation on them all at once, or a macro facility to apply common sequences of operations at difference times. If you can’t automate the process, then automate the user: design the UI so the user can do the task by habit with minimal thought. Walking is the same thing over and over (left foot, right foot, left foot, right foot), but it isn’t monotonous because we do it automatically. Include consistent, easy-to-use controls and tools that can become habits for practicing users (e.g., keyboard shortcuts). The idea is to get the user to the point where they can do something else while doing your task (e.g., listen to a music or a podcast, talk on the phone, check their email or newsfeed, or even watch a video).
Is it dull? That is, is it not sufficiently stimulating for the senses for the level of thought needed for the task? If so, you can try adding sensory stimulation, such as using brighter colors, or interesting animations, or background sounds. You could provide social stimulation, such as by having users linked by audio or chat. This allows them to discuss how best to do the task and motivate each other (especially if it’s structured such that they coordinate), but it also allows them to socialize to counter dullness. However, consider that users may want to fight dullness by providing their own stimulation (such as described for monotony), so allow them to turn off the animations and sounds, and make it possible to accomplish useful work on their own.
More details at Professor Q. Bert.