"It didn't place any limits on premiums increasing."
Not true. Part of the regulation is that insurers have to show that a minimum percentage of premiums is spent on healthcare costs.
Insurers must spend 80% (for individual or small group insurers) or 85% (for large group insurers) of premium dollars on health costs and claims, leaving only 20% or 15% respectively for administrative costs and profits, subject to various waivers and exemptions. If an insurer fails to meet this requirement, there is no penalty, but a rebate must be issued to the policy holder. This policy is known as the 'Medical Loss Ratio'.
So this basically means that when an insurer doubles its premium costs while the total claims don't rise, all the excess money will go back to the policy holder. Although this doesn't directly limits the development of premium costs, the effect is pretty much the same. As premium-payers will get a large part of their premiums back.
Also, if the insurance market will function properly, the incentive is to lower the premiums. Due to the new regulations, the incentive for insurers is to have as many policy holders as possible. And in order to attract those, insurers have to compete on premiums first and foremost. Especially when they want to attract healthy policy holders who tend to focus on buying the cheapest policies (because they hardly need care anyways, so the service aspect becomes unimportant).
If anyone is interested I could comment from the POV from a Dutch health insurer (which is my field).
Some background: http://en.wikipedia.org/wiki/Health_care_in_the_Netherlands
“As the Netherlands [is] expanding [its] lead among the best performing countries, the [Euro Health Consumer] Index indicates that the Dutch might have found a successful approach. It combines competition for funding and provision within a regulated framework. There are information tools to support active choice among consumers. The Netherlands [has] started working on patient empowerment early, which now clearly pays off in many areas. And politicians and bureaucrats are comparatively far removed from operative decisions on delivery of Dutch healthcare services!”