At the risk of being too venal: yesterday I had over 8500 and now I am under 7900. Anyone else experienced changes with similar orders of magnitude?
To slightly expand on Scott Morrison's answer, which indeed should explain most changes:
Another source for loss of points, which explains my decline of points (I never asked a question), okay not the massive decline that appears just now (but the one of the "true" acount), is that points gained from deleted contributions were (most of the time) never subtracted. This now happened as there was a recaluclation of reputation, and such recalcs will happen on 2.0 on a regular basis.
A possible source of gaining of points (though I am not sure about this) is that on 2.0 downvoting a question does not cost a point. It is possible that the thus "spent" points are retroactively refunded. Not sure though about this.
ps for OP of question: was this a "service question" or did you stop reading the old meta were I recently explained this (except for the final point which just occured to me now somewhat inspired by your surprise on the loss not being too big)? :-)
Two other sources of differences: The first 50 points in a bounty was contributed by old MO but not by the new version; also the new version detect repeated voting and discard them.)