I watched the evolution of Bitfinex right from start (disclaimer: not affiliated with the platform in any way). Initially, the similarities to Bitcoinica were striking. Then they reworked the site step by step and added and tweaked functionality. Especially the feature that on Bitfinex you use loans provided by other users (instead of just getting magically some leverage, as on Bitcoinica) was prompted by the user's constant worries about it being a "Bucket shop". Now, after this disruptive change it is pretty much transparent for every trade where the leverage comes from. Another side effect of that change was that the "magical" spread we knew from Bitcoinica was abandoned completely. Now you get just the actual market spread on Mt.Gox, plus the (slightly reduced) Mt.Gox fees, plus you have explicitly to pay for your leverage on a dynamic lending market. Bottom line is, it is not easier to make profit there (you still have to try hard and need a bit of luck and good timing), but at least now you can always see without doubt why you didn't make profit.
Regarding the possibilities to short: just look at the rates achieved for USD loans (needed to go long) and compare them to the rates achieved for BTC loans (needed to go short): Right now, no one really has much incentive to go short, since shorting just didn't pay off in the last months (which is reflected in the way higher demand for USD loans)
Regarding security, I'd say: yes, it is a new service. Thus use play money. It is a centralised entity and any web service can be compromised. But, please recall that Bitcoinica was not ruined by a technical vulnerability, nor was Bitcoinica ruined by an unsound financial construction. Bitcoinica was ruined by classical human mismanagement, the inability to grow a company in a healthy way, to organise a teamwork, the inability to conduct a business transaction for ownership transferral, the inability to speak with one voice, inability to communicate with customers effectively, so basically the inability to run a real company beyond a one-man show.