There are of course multiple systems of defaults, maybe one called "modern aggressive" with most doubles takeout and one called "modern conservative" with many doubles takeout but also many cards or penalty.
Here are a few auctions to test your agreements on (please add more controversial sequences if you like):
1. Berkowitz and Sontag disagreed (board 9) on this sequence: 1♠ P 1NT 2♣; 2♦ X.
Vugraphzfc said:
2. There was some discussion of the following sequence starting here. 1♦ P 1♥ P; 2♣ 2♠ P P; X. If the pass-and-then-come-in 2♠ offends, maybe 1♦ 1♠ X P; 2♣ 2♠ P P; X is good enough to consider instead.
Here is a poor attempt at the beginning of some defaults in the "modern aggressive" style, to show what I'm talking about:
Double of an opponent's suit bid is takeout unless:
1) Our side has already made a strength showing double or redouble (maybe this is supposed to be more restrictive)
2) We have a known fit
3) All four suits have been shown
4) The opponent's bid is artificial
5) Pass is forcing
6) Our side preempted
7) The doubler passed up an opportunity to make a takeout double earlier and is not balancing at the 2-level and no new suit has been bid
8) The opponents have shown 3 separate suits
9) The opponents have bid game and doubler has passed before
[suggested by wyman below:]
10) Partner has suggested the suit (even implicitly, as via a t/o X) as a place to play [note that this does not apply, for instance, to (1Y) 1N (2Y) X, where partner has shown cards in the suit but has not suggested it as a place for us to play]
I haven't gotten into cards vs penalty or more detailed stuff here yet.