Humayun's Tomb was the last of Delhi's WHS that I visited. It is undoubtedly the most picturesque of all of the sites, with its perfect shapes. It is easy to see how it has influenced the Taj Mahal. But I have not become a big fan of Mughal architecture here in Delhi: the buildings were constructed to impress, but lack fine details (or maybe these have disappeared during the ages, the glazed tiles are very few now).
My visit to the site lasted a mere 45 minutes, and I cannot see how I could have spent more time here (although part of the complex is fenced off at the moment). The "park" could use some more water, to make the lawns look green and fresh, and also to fill up the little canals and pools that surround the Tomb.
However, as Rob Wilson already stated below, the state of the site has improved a lot over the last few years. There's an exhibition on site where you can see "before" and "after" photos.