Learn Icelandic: Við höfum sama smekkinn, er það ekki?

This is a key rule for the identity determiner sami (the same). When you use sami, the noun that follows it usually must take the attached definite article. That is why we say sama smekkinn (literally "same the-taste") rather than just sama smekk.

Why is it sama here instead of the base form sami?

The word sami behaves like a weak adjective. Since the verb hafa (to have) requires its object to be in the accusative case, we need the accusative masculine weak form of sami. For masculine words, the nominative is sami, but the accusative changes to sama.

What happened to the -ur ending in the noun smekkinn?

The dictionary form is smekkur (taste), which is a strong masculine noun. Because it is the direct object of hafa, it must be in the accusative case. The accusative form drops the -ur ending to become smekk. Finally, the accusative definite article -inn is attached to make it smekkinn.

In English we would say "don't we?" here. Why does the Icelandic sentence use er það ekki? instead of matching the verb höfum?

Icelandic uses er það ekki? (literally "is it not?") as a universal, flexible question tag. Just like "right?" in English, you can add it to the end of almost any statement to ask for confirmation, no matter what the main verb of the sentence is.