Bitshares hasn't taken off yet because there isn't a working product yet, IMO. End users are not going to use a resource-intensive full client. They will use a web or mobile client. Only delegates need to run full nodes. We are very close to the end user solution.
I have used many exchanges: bitstamp, bitfinex, coinbase, bter, poloniex, mtgox, btc-e, vircurex, mcxnow. Bitfinex is by far the best as far as functionality and ease of use IMO. Maybe I can explain this graphically:
1) Direct exchange between assets. From what I understand, this is tricky to do but it is absolutely necessary to compete with centralized exchanges. A BTC/USD asset pair is essential and could be the focus to begin with for a real, working product. Bitfinex alone had over 1.2 million bitcoin exchange volume in February.
2) Advanced order types. These order types are very useful to traders. Coinbase exchange does not have these order types, only bare bones buy and sell, yet monthly volume in February was over 320,000 BTC - and this is only their 4th month trading.
3) Deposit/withdrawal. This is necessary and could go through direct links to on/off ramps such as metaexchange.
As others have stated, leverage is something traders absolutely want. Bitfinex uses 2.5:1 and OKCoin goes all the way up to 20:1. Bitfinex uses a P2P style lending between users. This may also be tricky to do in a blockchain-based system, but is necessary to compete. A P2P lending market may be the easiest to implement.
A simple web-based platform for buying and selling bitcoin and USD would be a good start. The key selling point would be that you are trading in a trust-minimized, blockchain-based environment with no third party risk. A
report as of April 2013 states that 45% of all bitcoin exchanges fail. That rate may be higher by now. If you percieve the risk of catastrophic systemic failure (where everyone loses all of their money) from bitshares at less than 45%, then the decentralized exchange is your best option, and bitshares has been in existence for nearly one year now without systemic failure in which you lose all of your money and that type of failure has never really been a threat.
This should lead to other markets being bootstrapped for trading. Eventually, the web-based interface could have a similar user experience to something like
TD Ameritrade, without the threat of third-party risk or censorship from using the system for any reason. End user security is very important with 2fa or something similar being mandatory.
Also, I don't like the rule requiring shorts to cover after 30 days. This was done to increase liquidity in the system, but may be taking liquidity from the system as users don't like the time limit on their trade, therefore they are being discouraged from initiating a short in the first place. I would like to see this rule removed completely or at least scaled back quite a bit.
tl;dr - We need to mimic the user experience of centralized exchanges such as bitfinex while using the bitshares blockchain as the backend to create a trust-minimized trading and asset storage environment. We should start by bootstrapping the BTC/USD market, which had a monthly trade volume of over 15 million coins in February alone. Just 0.5% of that market should be plenty to bootstrap the entire bitshares system with a healthy market cap and reasonable pay for developers. Then, more functionality can be added.