While it's true that some art styles are easier to extend than others, that's not what I meant by "quality". The artist commits rookie errors like using gradients on flat surfaces, inconsistent shadows, and not shading surfaces correctly based on light angle. This all makes things look flat and clashing.
Similar errors are committed by this pack:
http://opengameart.org/content/the-mana-world-woodland-indoor-tilesetNotice how the walls stick out much better than the benches and the armoire at the top-left, due to the shading (except the walls are still wrong because they're gradiented). The tables and chairs stick out only due to the shadows; the top-shading is wrong.
One other, more fatal error is that the characters are too low-contrast compared to the backgrounds. This is more obvious if you play a game that uses these tiles, as it's very hard to make the characters out. Compare to this:
http://opengameart.org/content/lpc-interior-castle-tiles where the character really sticks out due to bright colours and hard outlines, plus it doesn't commit the shading errors, which means the furniture and stairs don't look flat at all. LPC has other problems but overall it's a much higher quality pack.
This means that the set looks good from a distance - it's colourful and has lots of variety - but once you start using it in a game the problems become apparent. If you have access to an artist with any formal training at all, you'll be able to produce better art than this. Sometimes you don't need an artist at all, just a cursory understanding of visual theory. As an example, and this will seem biased, C-Dogs art gets the shading right despite being drawn by a programmer. So use this set for prototypes but be aware that as final art, it's simply not up to snuff.
When you create art for games, you can't just pick colours because they look good to you, you have to keep in mind how they highlight certain gameplay elements, in addition to the basics like how light works, foreground/background composition and shading to make things look solid. I'd expect the visual hierarchy to be something like: characters > furniture > walls > ground, whereas this set is more like walls > ground > furniture > characters. Take a look at this article
http://www.gamasutra.com/blogs/HermanTulleken/20150729/249761/Color_in_Games.php, near the middle it talks about various techniques you can use with colour to highlight things.