One of the biggest was that there was an investment tax credit of 7% in the 60's and 10% in the 70's, which I believe was uncapped. I think those were also able to be rolled over to cover multiple years. That means that if you invested enough, you could pay essentially no taxes. This was repealed in 1986.

[edit] Another that I should probably mention is the treatment of asset depreciation; in the 1960's and 70's the government was incredibly generous with regards to asset depreciation. For example, they allowed more rapid depreciation so you can have assets fully depreciated while still within their useful life and potentially if you sold those would pay capital gains tax on them, meaning you reaped significant tax savings over the asset's lifetime.