Futures would make the Bitcoin price much easier to manipulate. One major banking institution or other wealthy group could manipulate BTC users in pretty much any way that they want.
Since BTC has less of a clear value than most other stocks and assets, it means that investors could be manipulated to push the price as high (or as low) as they want.
However, I suppose that introduction to a more mainstream and regulated audience could be helpful, as it could theoretically free us from the emotionally unstable investors we have now.
This is my concern as well. Futures worry me like hell, they are so open to manipulation. But big players entering the market is always a good sign on the other hand. If this move helps bitcoin expand in terms of the number of people owning the coin, i.e. helps decentralize it more, then I'm all for it.
I think the fear is that there is a chance that these big players you mention entering the market would buy up large positions of BTC and control the market. It would actually have the opposite effect of decentralisation and continue the trend of large institutional investors controlling financial markets.
My big thing is that bitcoin is already highly speculative and bringing in instruments that encourage more speculation will make trading even more volatile for the average consumer. Honestly the crypto market is just going the way of traditional financial markets and soon I think the whole ethos of Bitcoin and others will be lost to that pesky human attribute called greed.