1. The end goal is security. Fairness or representation in my opinion are not of value here. Everyone can participate which is fair enough. In the end, voting participation will depend on how easy it is to vote (client and voting / delegate infrastructure will have to get more user friendly) and on how much stake one has.

Ok, let's put this on hold. When I have time I'll show how this could be used for an attack.

2. Maybe I understand what you mean... Would you agree that in the same sense Bitcoin miners and NXT forgers also have that control over the blockchain (=communication channel)?

No. Because more miners can join Bitcoin and NXT forgers get delegated power expired eventually.

3. User arhag made a great post on that https://bitsharestalk.org/index.php?topic=6638.0
You could challenge him in the above thread.
BM also made a post about such attacks http://bytemaster.bitshares.org/article/2015/01/08/Nothing-at-Stake-Nothing-to-Fear/

Will do.

I'd like to see 1. and 2. elaborated. The third is a weakness that's somewhat inherent in the system.

Look at https://bitcointalk.org/index.php?topic=940298.msg10388364#msg10388364 for #1.
#2 is based on http://en.wikipedia.org/wiki/Falsifiability. This concept is quite sophisticated, but common sense says the same - if delegates censor communications then they can remove what they don't like.

There was my thread about hypothetical attacks on BitShares somewhere. Those attacks were technical ones, targeting implementation of BitShares. In this thread I'd like to list potentially weak spots in BitShares concept. I don't provide very detailed descriptions, it all was already discussed on BitcoinTalk. If short description is not enough - ask me and I'll explain the details.

The goal of this thread is to write down the info somewhere, to get peer review, to be pointed where I'm wrong and maybe to help to improve BitShares.

1. Delegate election is not very "democratic". The top delegate - market.cn.group101 - got only 16.71% according to https://bitsharesblocks.com/delegates. The issue can be completely or partially solved with another voting approach.
2. Delegates control the only mean of communication available to bots and smart contracts. The same problem as in a state that controls all mass media arises. Humans can use other means of communication (e.g. this forum) but DACs don't have this option. The issue can be solved by introducing something leading to "separation of powers".
3. Delegates validate blocks one by one. Several branches of the blockchain may exist and compete against each other. Voted-out delegates may generate a fake blockchain to trick newly connected nodes. Something similar to Economic Clustering (Nxt) could solve this issue completely. Validation of every block by 51 delegates may solve the issue of competing branches.

These are 3 main points. There are some more but I need more time to analyze them and make sure that BitShares indeed work as I understand it.

sry dont get you! You mean bitshares toolkit, right ?
If so yes - https://github.com/BitShares/bitshares

I thought DACs are impossible without Smart Contracts. Do BitShares have Smart Contracts already?

Dan did a really good interview on bitshares.tv about DAC's..
https://www.youtube.com/watch?v=abA4TuPHmNg

Thank you. Unfortunatelly, I learned English from programming languages documentation. I don't comprehend spoken English in 99% cases.

I tried to open the link to BitShares DAC whitepaper (from http://wiki.bitshares.org/index.php/Main_Page) but even after logging into my Google account I wasn't able to get the access...

Assuming sybil attack is possible, is an eclipse attack feasible?

Yes. Most of "DDoS service" packets include "100% efficiency or money back" condition. So seed nodes won't help much.