Ideas behind the 8.Bd3 line in the 4.Ng5 Two Knights Defense In the Two Knights Defense, after 

[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]
1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 Na5 6.Bb5+ c6 7.dxc6 bxc6


8. Bd3 has become fashionable recently, and is considered a serious way for White to play for a win. 

Yet, is seems an artificial move, going against basic opening principles, in particular blocking the d-Pawn. What are the main points of this line, especially in comparison to the "old" 8. Be2 move, and are there any typical traps or pitfalls to be aware of?
I think the main reason behind Bd3 is to provide a "safe" retreat for the knight and keeps to e-file open.  Both Nf3 and Nh3 have liabilities. A retreat to e4 also allow a further lose of time from f5, but this may be offset by black's LSB having less mobility.  The other option is Bf1, which while playable, also goes against basic opening principles.

Although this may apply here, paraphrasing Alekhine, to punish unconventional moves (Na5) you may have to play unconventional moves.