Well ur rite, a good camera doesn't only depend on 1 single component....a modern digital camera actually has 3 major component that i think is the most important in determining a good camera....
1.) The Lens - this is the major factor that affects the quality most...a good lens will make a lot of difference in terms of picture quality, sharpness, color and most importantly...bokeh or depth of field....
2.) The Sensor - this is one of the most misleading or confusing information that has confuse consumer for a very long time..i think most of u guys already know it but i'll just say it anyway, more megapixels doesn't mean better quality..it all depends on a sensor size...for example a 12mp - 1/1.6" sensor will not look better than a 6mp - 1/1.6" sensor...this is due to the large amount of photosites they need cram in a small space...and the only way to cram it is to make it even smaller than they already are!!! Photosites is used to catch light in the sensor and then convert them into electricity thus making them into a digital signal...small photosites are not as good as catching light compared to the bigger photosites in a larger sensor found in D-SLR..(mind you that 1/1.6" sensor is only around 8 x 6mm while Nikon DX format is 23.7 x 15.7mm...that's several times bigger than 1/1.6" sensor), because the signal that the small photosites gather is weaker they have to bump up the signal to get a better picture, but when u bump up a weak signal then the noise gets bump up as well, so thats where all the noise come from..another thing that i don't like about small sensor is the ability to control the depth of field..if u notice, a compact camera doesn't produce as much bokeh as D-SLR does because of the sensor size..so big sensor big bokeh, small sensor limited bokeh
To tell the truth i'm actually sick and tired of this megapixel race where they put as much pixel as they can in a small sensor without considering the consequences. so please forgive me if i don't have anymore confident with a large megapixel,small sensor type compact camera

...
3.) The Processor - well the first one to brand and promote a camera processor is Canon i think, with their DIGIC processor..so what do processors do? the sensor read the light and then converts them to signals, then who convert the signal back to a picture...the processors la...the processor(lets call it CPU) is responsible for constructing the signal as well as managing the picture quality..the CPU actually controls the white balance, color saturation and most importantly noise reduction (these r only some of the things that a CPU controls)..a same sensor on a different CPU will yield different results. even with the same sensor and the same CPU which has slightly different algorithm will produce different result also, in the end it all comes down to what the manufacturer sets them to be..some people like to bypass the CPU by taking raw picture files and then calibrate them using a 3rd party software, eg:Photoshop...
So sorry that this might sound like a lecture but i just want to express whats on my mind for a very long time..
the Sigma might have crappy functions and might not produce an ideal looking picture to u but its mostly due to the processor fault (they are not very good in that department anyway)..and it takes a long time to come out with a good processing algorithm to control the overall quality of the picture.... i hope i didn't make any mistakes and hopefully this will provide some info to anyone wanting to learn more about photography.Thanks