Nearly everyone has a "to-do list," whether it is formally written down or simply tucked away somewhere in our brains. People who are active participants in society always have at least a mental list of things they must do at home, at school, at work, or wherever else they have obligations. It is not uncommon to have one or two items on that list for days, weeks, or even longer; these are the things, for whatever reason, we just do not or cannot accomplish. Your question asks what the differences are between the items which are easily crossed off the list and those which linger until we finally either do them (or just quit putting them on our lists).
One reason some activities are more difficult "to attend to" is because they are time-consuming. Life moves quickly and time is a valuable resource; sometimes the larger tasks will consume more time than we can spare in a day, so the task goes undone.
Secondly, it is much more satisfying to be able to cross four or five smaller items off the list than just one item, so certain things just stay on the list. Sometimes it is more important for us to feel as if we have accomplished many relatively unimportant things than to have completed one meaningful task.
Another reason some tasks are difficult to accomplish is because they are not things we like to do. Even a difficult task will get completed if it is something we enjoy doing; however, a relatively easy task can be left undone for days because we simply do not enjoy doing it. This will, of course, vary by person since we all enjoy and dislike different things.
Some activities are perpetually on the to-do list because they take more resources than we have: money, supplies, help, proper instructions, emotional energy. While the task itself may not be particularly difficult, if we cannot do it alone or if we need things we do not have, it can seem like a task best left for another day.
Finally, the task that is perpetually left undone may not be a realistic task for us to accomplish. It is a good thing to wish and dream; however, putting a wish or a dream on a to-do list is not likely to yield results.