same feeling here about Binding of Isacc, it adds a bunch of content, but it's not necessary to enjoy the game, esp since the game requires so much time without the DLC. In fact, because the DLC makes the game harder, I would honestly say that you should play it without the DLC first, until you've unlocked a good number of the items so that you actually stand a reasonable chance.
There are other "indy-ish" games that have DLC that do it right though. I really liked how Killing Floor only had cosmetic DLC (until the most recent DLC set that is). So you never lost out by not buying DLC, but if you wanted more than basic customization it was a couple bucks. I'm less happy that they released a "weapon pack DLC", but at the same time the guns don't remove playability or make it impossible to get any achievements, they just add some silly options to the gameplay, like a buzzsaw crossbow, so i see that as more acceptable.
The DLC for DD and Magica piss me off, because they serve the same function map packs for big AAA titles do, which is to largely segment the playerbase, or nickle and dime you.