A variable based on keyword has already been reported being a factor - some are easier to work with than others - but age of site seems very much to be a factor even where the same keywords are being targeted.

I have an example of that in action here: Google and Age of Site where I show two similar site rankings side to side. Even the sandboxed site is able to rank for some less competitive keywords - but is unable to compete with an older domain where they share the same target keywords.

Barry Schwartz also references age of site here when using the DP co-op network:
DigitalPoint Coop Ad Network and the Sandbox:

I do not believe that the Sandbox has to do with links, themselves. People feel a new link is sandboxed. Also people think that a new URL is sandboxed. I think the sandbox goes on the domain name. I have tested the other variables and strongly believe its a new domain name issue.

I have sent links to new domain names with the DigitalPoint network and did not rank well for those keywords. Then I created similar pages, targeted the same keyword, but on an existing domain name (unsandboxed name) and I achieved top rankings for that keyword.

Rand Fishkin has also made a point of bringing together a lot of the ideas on sandboxing here:
Potential Solutions & Updates to the Google Sandbox Theory

Scottie at HighRankings suggests that sandboxing is further complicated by an aging delay factor:
Sandbox And Aging Delay

Of course, I also have a collection of online notes about the Google Sandbox.

I believe that papers such as Hilltop and LocalRank suggest ways in which to create a subset of popular search phrases for ranking purposes, and it wouldn't be surprising if a subset based on keywords were one of the variables involved in the Google Sandbox itself - but it only appears to be one of a handful of factors.