Finished the last two worlds today (5 and 6, unsurprisingly), and then defeated the final boss that comes after them. The credits rolled, and the game was over, but then…
Then two new modes were opened up – Expert and Plus. Plus is a bit like a cross between the standard levels and the mini-Mario levels, whereas Expert is just a load of difficult levels. Had a go at a few, and may well plough through some more, but for now the game is complete!