Aditya,
Here's an example (using 2014.02.14 beta). The "Sun" subscript using _{\tf Sun} or _{\rm Sun} was in Palatino in the corresponding MkII setup, but is in Euler now. The workaround of an hbox with \tfxx or \tfx doesn't get quite the right size.
That is because in math mode, \rm is symonym with \mathrm etc. The standard way to write this is _{\text{Sun}}.
I agree about \rm. However, _{\tf blah} once (in MkII) selected the text font. e.g. \starttext \placeformula\startformula M_{\tf Sun} \stopformula \stoptext Is the II->IV change in \tf semantics intended? It's not hard to adjust for by using 'sed'. I raise the issue only in case the change is not intended. Actually, the example just as above (i.e. using modern fonts) works fine works in MkII and MkIV. It also works if "\setupbodyfont[palatino]" is inserted as the first line. It's fails only when using Palatino with Euler (at least, in the way that I did it). That data doesn't say what the right behavior is. But it may help you decide. -Sanjoy