I am very skeptical about the security of the proof of stake system. Is it really that secure? Are there no disadvantages?
Vanilla proof-of-stake doesn't achieve consensus, unfortunately. People staking their coin can vote for both forks of the blockchain, and can even mine effortlessly in secret. In PoW this is impossible, as you are literally wasting energy by mining both sides of a fork.
To mitigate this, most modern PoS schemes use a mixture of PoS and PoW to ensure that someone will finally decide the chain.
http://www.reddit.com/r/Bitcoin/comments/1oi7su/criticisms_of_proofofstake/ for more discussion.
Proof of stake has been used for many months in Nxt, and has not been broken despite having the third highest market capitalisation, so it does seem to be secure. If it wasn't, someone would have broken it by now. History shows forks rarely persist for more than a block, so it does achieve consensus.
In theory, a weakness is that people can vote for both sides of a fork. In practice, that doesn't seem to happen. The gain from doing so would be trivial (there's no block reward in Nxt), and the loss of security substantial, so no-one does it. With Proof of Stake, the people who secure the block-chain the most are also the people who possess the most coins, so they have the most incentive to preserve the integrity of the currency. To put it another way, in Nxt forging is done to secure the block-chain, not to make a profit.
One disadvantage is that in pure Proof of Stake, the only way to acquire coins is from someone who already has them. This can lead to issues with the distribution. For example, in Nxt the entire coin supply was initially distributed to 73 "founders", and some of those people still own significant fractions of the supply, giving them wealth and influence. That said, none now own as large a fraction as Satoshi owns of Bitcoin. It seems that the distribution problem solves itself over time, as the founders have an interest in spending their wealth to support the coin.
the major one I can think of is the stakeholder with high stakes can have more easy execution whereas stakeholders with low stake will have to get less limelight to overcome this we have delegated proof of stake in which coin holders get to cast their vote and they may also call upon for network change parameters
For the offline stakes in the pos, they are highly vulnerable, to prevent them from attack we can have proof of checkpoint which ensures miner between blocks and matins scalability
There's an argument that certain properties of cryptocurrencies encourage hoarding, but I think this is wrong. If someone ends up rich through 'hoarding their coins, there is then an incentive to spend some of those coins, to buy a car,a house, go on holiday. I think this has been levelled at POS coins too, that eventually, all the coins end up in a few hands of big holders. But they are going to spend at least some of their wealth on a luxury lifestyle. Probably, they will remain super wealthy, but, their wealth will always be being spent and that wealth passed on to others, eg the guy who built the Aston Martin he just bought.