It depends on the database you are using, because there are databases like postgresql that can have arrays or complex dataobjects as datatype in their columns. It normally comes with a price: you can not index nor organize this data. Then might it for instance become difficult to find out which comments an user has given on which posts. If you take a look at your own Stackoverflow.com profile, then will you see the benefit of keeping this information separated.
You can remove one table however. If you create a foreign key to the post_id in the comments-table then will you have one table less in your design yet you can make use of indices just as you need.