A lot of people moaned about the lack of interiors in GTA IV, I thought it had a decent amount. Law Firm, Museum, Bank of Liberty, Subway System, to name a few. Plus there were a lot of random apartment blocks, flats, and houses that you could enter around Liberty City - most of them having no mission relation. One thing I did notice is that for whatever reason they never included Grand Central station, although if you go to the doors, you can see that they were intended to open, and if you remember, it was featured in the first trailer.
So as long as V has the same amount of interiors as IV did, relative to size minus the country side, I would be more than happy. I guess around twice as many interiors as IV. One thing I would like them to do though, is vary the interiors a bit more. In IV you could be in any of a hundred flats and you would have no idea which one it was from the inside. Also, interiors with escape routes are important, places where you can hold up a rampage, and then break out some back door or something.