Not only do you lose the community as already mentioned, but you lose the accumulated data (i.e. post content). This might not matter for a social community; it matters a lot if the community is of a more technical nature; i.e. repair/modding, analysis, etc. A lot of really great and useful content has been lost over the years due to forums going dark; lose a site like reddit and... my god. It's just really a huge step backward.

N.B. I have no love for reddit. I generally hate reddit, the site. However there's a massive amount of useful content there.