Open core is usually built by a set of internal open source components held together by a dual-licensed wrapper, plus proprietary modules on the outside. One of the best examples of this is Zimbra (an excellent product on its own) but MySQL in recent editions can be included in the same group. As discussed in previous posts, dual licensing hampers contributions because it requires an explicit agreement on ceding rights to the company that employs it, in order to be able to relicense it for the proprietary edition. This means that Open Core companies, in itself, will have an easier time in monetizing their software, but will receive much less contributions in exchange. As I wrote before, it is simply not possible to get something like Linux or Apache with Open Core.
Again: open core is not bad per se (but I would have been more cautious in calling Sugar “an open source company”, for whatever definition you have of that). But it is a tradeoff: monetization versus contributions. And, my bets are on contributions, as OpenStack demonstrates – you need leverage and external resources to go beyond what a single company can do.