There are main theses from this document:
Removing stale orders: Bittrex will be removing orders that are older than 28 days;
Raising the minimum trade size: the minimum trade value for orders will go from 50,000 Satoshis to 100,000 Satoshis;
Creating minimum tick sizes: Bittrex will be instituting minimum tick sizes. The target is to have minimum trade sizes that are near 0.1% of the current price;
A general statement about market manipulation tactics: Bittrex actively discourages any type of market manipulation, including pump groups. They will suspend and close any accounts engaging in this type of activity and notify the appropriate authorities.
All these parts will come into force within the next 2 weeks.
Bittrex users already are afraid of incorrect application of the last measure specified in the letter. They think that this is a too blurred criterion under which any user can get. They also fear that if Bittrex decides to block these accounts, users will lose their money.
That massive problems with verification, then at any time you can be considered a manipulator, a fraudster and block your account with all the money you've got.
user of bitcointalk.org forum
Other changes are not seemed so fatal to users. For example, they think that creating minimum tick sizes will reduce the number of bots. Then this letter provoked the appearance of complaints about the work of the stock exchange as a whole.