Dobsonians may need to be collimated before being used at high magnifications, especially the larger ones. Over time, their mirrors may get dirty, but cleaning should only be done very, very rarely, only when the images are actually noticeably worse. It's better to use the dust caps properly and prevent dust and dirt from getting on the mirror to begin with. On some models, the bearings can come loose over time. Should be nothing a screwdriver can't fix. Because Dobsonians are so simple, there's not a whole lot that can go wrong.