Popular vs. Unpopular
As of March 2011, support for the repeal of ObamaCare is at 53%, while opposition to its repeal is at 42%. According to Rasmussen Reports, support for repeal has never gone below 52% and opposition has never reached higher than 44%. This may sound like the country is evenly divided on the topic of repealing Obama's health care legislation, however, it's worth noting that the majority of respondents has always supported repeal since polls have been taken. On the other hand, 84% of Massachusetts residents are satisfied with their health care system.
Little vs. Big
This difference is also very obvious. RomneyCare, as a bill, was less than 100 pages. ObamaCare, in bill form, was over 2000 pages. While both centered around an individual mandate to purchase insurance, it's difficult to see how RomneyCare could have been nearly as complex as the 2000-page bill that ObamaCare became.
Partially-vetoed vs. Unvetoed
The likelihood of Obama vetoing any portion of his signature achievement in office seems quite small. But as governor of Massachusetts, Romney vetoed eight portions of RomneyCare, which were promptly overridden by the mostly Democratic state legislature. The case could be made that the less conservative portions of RomneyCare would have been eliminated if Romney had had his way in the matter. This is probably why he has recently said that there are things about RomneyCare that he would change. Furthermore, Romney has stated that if he were president, he would repeal ObamaCare.
I could go on and on about how RomneyCare didn't raise taxes and how ObamaCare did. I could elaborate more on the stark contrasts between a federal plan and a state plan. But the important thing to take note of when comparing the two plans is that their differences far outweigh their similarities. Anyone who would say otherwise is clearly taking aim at a perceived political weakness in Romney, rather than laying out the facts.