They've got a huge selection, reasonable prices, lots of interesting nooks and crannies, a piano, a chapel, a warehouse with dozens of boxes of books you're welcome to rifle through, and a boa!
... Seriously speaking, though, I like what they've done with the system after the move a few years ago; most of the stock is on the shelves but there's also stuff in boxes, easily available for perusal, and organised by genre/category/language. There's a pool table available for a small hourly fee, and also tea and cookies and whatever happens to be there the day you walk in. They hold numerous events at the shop, all of which are listened on the page, and the owner is quite possibly the friendliest antiquarian in Helsinki.
And by the way? I wasn't joking about the boa.