A major criticism on DPOS is its small size of producer group, currently 21 in Steem and 25 in BitShares. As @iang mentioned, "How large is large" is always the question and balancing between more decentralization with inefficiency and more efficiency with centralization is hard to achieve. However, given that centralized hashing power distribution (e.g. Ethereum's top 20 miners consist 99.4% and Bitcoin's top 20 has 99.5% of hash), our opponents are not free from this "smallness" issue as well.
On the other side, there is another criticism that DPoS gives too many votes larger than a producer pool size (e.g. 30 votes for 21 witnesses) and therefore it opens collusive voting for whales and centralizes power. Technically, this is not true since whales can split stakes with small number of votes. However, I think this still matters from the perspective of cognitive burdens that humans can deal with, so-called the Dunbar's number.
Generally, Dunbar's number is known as 150, which human can maintain relationship comfortably. But in his article "Discrete hierarchical organization of social group sizes, various sizes by hierarchy was suggested. More specifically, people can have 30-45 of "close friends", and 9-15 of them are "intimate friends", and 3-5 of intimate friends can be called "best friends".
Then, what does 30 of Steem witness votes mean? It means you need to use third level of cognitive burden. To manage and closely monitor 30 witnesses, voters must pay greater efforts than have 15 or 5. Surely, people can only vote for few witnesses. However, it is also true that people can vote 30 times without prudence, or even they can use the leftover (meaning they already voted for whom they value but there are more votes) for vote trading.
There should be additional incentives to differentiate active voters who are willing to use their 2nd or 3rd level of cognitive energies. However, this incentive must not exist too much. In my opinion, a better decentralized governance can be achieved when as many citizens as possible are acknowledged and act in favor of the platform. For this, I suggest to reduce witness votes to 5-9, which is in between the thresholds of best friends (3-5) and intimate friends (9-15). This change will make active producers into multiple groups according to voter's interests. With the smaller number, voters are able to pay more attention to all producers they are voting and more actively response to producer's performances/stances.
Relative posts:
Seeking Consensus on Consensus - DPOS or Delegated Proof of Stake and the Two Generals' Problem
DPOS system for Steem ?