I'd sooner look at ways the system can be improved, rather than scrap it altogether. For example, increasing the amount needed for a comment to be hidden. Or removing the anonymity attached to up and downvoting.
The theory behind the increased voting power for people who donate to the site is that said people are more likely to be good, upstanding members of the community who want to improve the site, and so are likely to only downvote posts that are genuinely deserving of it. Not everyone, sure, but the vast majority I'd hope.
I don't think donations have any relation with being a good member if they are getting something back for it, at all.
I understand the premise, helping the comunity is a good act, but not if it's money buying it.
If you are donating time, actual support and helping, sure you are likely to be a good (or at least better than usual) member of a comunity, but all about money is not personal, simply giving money for something doesn't mean anything, even if it is a lot, money help but money doesn't make anyone better.
That's my take on it, and I know you should have incentives to ask for donations, but that's already telling itself, you are selling influence, aka power, for money, so yeah, I really disagree that it have any relation to the personal quality of anyone donating it.
No opinion should be numerically more valuable than another, and if someone wants to donate that's cool, that medal like tag system to show it is already enough.