Since there is no real knowledge of the orbital parameters of the Tunguska impact object, let's say it impacts in 1928, and strikes Moscow. What would the effects of the loss of Moscow, and Stalin, be?
Why don't it strike Warsaw?

Seriously speaking, it is impossible to forecast leadership changes without exact knowledge of the hit list. Who survived and who perished. You've only said that Stalin is gone. Without him, the struggle would be between Trotsky (who survives by virtue of being in Kazakhstan exile since January 1928) and Kamenev-Zinoviev-Bukharin-Rykov group (each of them isn't powerful enough to oppose Trotsky on his own, but their combined clout would be significant, if they all survived).
Speaking about economy, impact would be serious, but not critical. Judging by photos of Kulik expedition, I can assume that transportation network would be mostly intact (some bridges might come down, but Moscow isn't exactly in mountain country, so potential damage to railway system will be fixed fast; besides, I believe that there was "Ring Railway" at this point already). Factory buildings would be gone, but most equipment will be salvageable (there's no radiation poisoning to worry about). Moscow population in 1928 was hovering around 2 mln. How many of those would survive is a guess.