Is it bad to leave my mac on (not just asleep) constantly?

I leave my mac laptop on a lot (almost 24/7) and even go to sleep skyping with my friend all night. When going somewhere I usually just shut my laptop and pack it up as well and want to know if this damages the harddrive and what I should do. Skype has been acting a bit weird and my internet connection was also out of wack the last day or so as well when it hasn't been an issue before (could be due to using skype and firefox at the same time but I am wondering if it should be able to handle that or not as well). The internet is fine but it is running slow and doesn't seem to be working correctly. I have already had a harddrive fail before due to a lose wire or something and don't want to go through that again.

What kind of things would shorten the lifespan of your harddrive and how much wear/use can they handle before they start to wear down? Also, how many applications should be able to run at the same time without things going slow? Is that an internet issue or my computer? If it is my computer then what is the best course of action I should take?

Thanks