The more advanced filtration systems on higher end tubs will keep your tub water cleaner longer. Clear water doesn't necessarily mean clean water.
Advanced insulation techniques will reduce monthly operating costs. Regardless of thermopane or "full-foam", both can work when done correctly. On Hot Springs models (which I have) they estimate a monthly cost of about $18-25. So far, knock on wood, that estimate has been just right. I have heard others say that, in the winter, cheaper spas incur operating costs of up to $60/month.
The number of pumps, quality of the jets, integrity of the shell, insulation from vibration, energy usage, plumbing quality/pressure/efficiency and ease of water maintenance will be drastically different between a top of the line manufacturer such as Hot Springs and a lower end model such as an ICON.
The warranty that comes with other models is also, I would imagine, a large part of the price. Most expensive hot tubs come with at least a 5 year warranty on all moving parts and all plumbing and usually longer warranties on the shell and cabinet. Hot Springs (watkins) has a good warranty and a real history of sticking with it especially through a few growing pains they had many years ago. They have a great track record with servicing tubs.
Also consider how you will install the tub.
An ICON tub will probably have more problems but if you are handy and so minded you can open it up and fiddle with it. If you have it flush-mounted in a deck, though, you may want to spend more money to have less "fix it" issues and save some trouble trying to access tub parts from weird angles/spaces.
-Ed
