In my opinion there are many things that go into a "good" install as opposed to a standard or mundain install and they are as follows
1- how much time you spend in the details. Did you take the time to buy the best wire best RCA's you could. Did you spend the time to buy the right connectors for what you are hooking up, or did you just splice into wire and use electrical tape? Did you take the time to hide all the wires and tuck them into where they would be best suited? Did you run any of the wire through the firewall with a gromet or did you just push it through your steering boot? Those are big to me.
2- Did you take the time to set up the stereo components? That is did you match your amps to your subs and you components or coaxiles, or did you just buy an amp that has plenty of power to push them? Did you set the gains with precision or did you just eyeball it and pray? Did you buy an equalizer and set the settings on it the way that they should be for your perticular car, or just again "eyeball" it or play it by ear.
3- Did you take the time to build the box specific to the subs that you have or did you just buy a prefab and make due? Did you Carpet the box? Did you take time to build the box into the car and the space available or did you just build the typical square or rectangle box and say the hell with it?
4- Did you take time to upgrade any stock components in the electrical system, i.e the alternator, battery and second battery, the big 3 upgrade with the necessary size wire? Did you Take the time to check voltage dip and regulate the output voltage of the alt to adequately support the core electrical system of the car. i.e the car's computer and dash.
5- Finally how it looks and how loud it is.
I would like to say however there are different installs. you have SPL and SQ and in those you have to adjust the order in which I listed these items. I would not be as concerned with certain items in a SPL install as I would in a SQ car.
I don't want to say that prefab boxes and Line Drivers and equalizers are bad or even necessary in all installs but they are in many. You can't often find a prefab box that is perfect for your install. Just as though you don't always need the LD or the Equalizers, but in most cases they do make the difference. I also am not saying that everyone needs a second battery the big 3 or the HO alt but in many cases it does make the difference between an okay system and a good system. It really all comes down to the attention to details. Focus all the components properly and fuse your wire correctly with the right size fuse, all components of the system (subs, box, amps, any other speakers or wires) are properly installed and secured. Sound deadening is used properly and in the proper areas. All items installed have a purpose and are used correctly. That makes a good or great system and it will show. The best way for this is obviously Knowlege! For this there is no substitute, and that just takes time. Ask as many questions as you can and you will learn, but you really need to hands on. You can get this by both actually doing it and drawing it out in it's entirety on paper. All wires both pos and neg and where they will be run and the system components and where they will be located in the project vehicle, then have someone that knows what they are doing look it over to check your work, and when you run into problems or are not quite sure, ASK!
Hope this helps. and Good luck with all those making an attempt in a good system