Base maintenance can be dull. Doing the same checks week in week out on the same type. It can also be very rewarding when you find something completely unexpected and end up with a major repair or engine change etc.
Line maintenance can also be very dull, but to be honest if it's been a bad week with every a/c that comes through falling over then dull is what you need for a couple of weeks. In reality Line maintence tends to be like fightng fires in an oil field, you put one out and another crops up sooner rather than later. You also have to deal with some rather unpleasant individuals who no nothing about aviation but assume that they know best. The ability to bite your tongue when faced with career management types who have zero technical background and think they know how you should do your job is an asset.