A high end vitamin tries very hard to package nutrients from actual food sources (i.e. extracts and concentrations of something you would really eat) instead of just cramming in raw materials. This tends to mean that the bioavailability (i.e. how much you actually absorb instead of just pass through) is much better. For example, sourcing calcium from ground up oyster shells provides plenty of the mineral, but the absorption rate is terrible. Much research goes into finding the best bioavailable forms.
In general, the ingredients are significantly better quality. Packaging B12 as cyanocobalamin may cost 1/10th as much as hydroxlcobalamin, but isn't nearly as good for us.
The ingredients are plentiful. The FDA's recommended daily allowance numbers are WAY off in most cases. For example, my multivitamin packages over 3000% of vitamin C, and over 8000% of B1.
Finally, a high end multivitamin includes a huge range of stuff beyond just the basics. For example, look at the ingredients list for the LEF Mix Tabs. It's 4 pages long in my browser. If you're curious, I suggest Google or Google Scholar around for the specific chemical forms of all the vitamins and minerals mentioned.
Most of them want to create the perception that you can "feel" the results of taking the supplement, so they contain stimulants like gotu kola, synephrine, etc. Some of these, like synephrine definitely are not safe (raise your blood pressure... small doses of that get me to 180/115) and other ones are questionable.
First, nearly all vitamins sold at CVS, Walgreens, Costco, Walmart, etc. are cheaply manufactured. Don't even bother. These are also the vitamins pharmaceutical companies study when they wish to discredit the healthfulness of vitamins in general.
Next, health requires diet, exercise, and supplementation for nutrients that can't easily be found in our modern food supply. Supplements alone won't do it.
That said, the best quality supplement companies I'm aware of are (in descending order for price and quality) Wellness Resources, the Life Extension Foundation, and Vitacost. Prepare yourself for sticker shock. The best protein supplements I know of (fermented or sprouted brown rice base, no animal or soy sources) are from SunWarrior and Boku Superfood.
The supplements I use are mostly plants, and include chlorella, spirulina, tumeric + black pepper, CoQ10, resveratrol, and LEF's Mix Tab vitamins (9 per day; a wide range of things are included.)
I also include flax oil, maca root, and pure chocolate directly in meals, along with an abundance of colorful fruits and vegetables, nuts, and occasional good quality meats.
Previous comments on supplements and diet:
http://news.ycombinator.com/item?id=1007392
http://news.ycombinator.com/item?id=829701
http://news.ycombinator.com/item?id=726914
http://news.ycombinator.com/item?id=585575
http://news.ycombinator.com/item?id=516538