I do happen to carry both a World MC and a Visa Signature, but that's likely as far as most people can get. The World Elite Mastercard is in a realm of its own.
The World Elite Mastercard and Visa Signature are presently the top of the prestige tier for MC and Visa, respectively, developed to compete with Amex platinum. Countering that, Amex shuffled out the Centurion (Black) card, with its nightmare annual fee. The response by MC has thus been to partner with only a couple of the most affluent stores (for now, at least) and cough up the World Elite Mastercard (another black card). Visa has not yet released its own black prestige card, considering Signature to be the workhorse for now, but they may still do so in time. Discover does not seem to have opted for joining this competition yet, but that may be because they realize Visa and MC have a world market while Discover is generally limited to the US (as is Amex).
However, these 'prestige' cards, which all offer open accounts (i.e., no preset spending limit), are now being offered to larger percentages of the population, which to me indicates the following move: Platinum by name is joining the Classic/Gold tier, World and Signature for those who have excellent (FICO 720+), established (10+ years) credit (the "new" platinum), and World Elite and Centurion for those who have six to seven figure incomes in addition to excellent, established credit.
Another thing about these cards is that their benefits vary by issuing bank. Although they seem to all share stuff like the no preset spending limit and travel insurances, the more elite services (such as Concierge (Mastercard) and Dining (Visa)) generally are not tagged onto cards issued for those making less than ~$125,000 per annum. You can check your particular card's status for these at
http://www.priceless.com (MC) and
http://www.visa.com (Visa).
Most travel-based World and Signature cards carry an annual fee, though their lower platinum equivalent is usually annual fee free. If you're looking to focus on a specific airline, these are good. However, some general rewards-based World and Signature cards ARE annual fee free, some examples being the Chase Freedom World Mastercard (whether points or cash back, for points, 3 points / $1 in top 3 categories, maxing at 1200 points) and the Capital One NoHassle Miles Visa Signature (1.25 points per $1).
Interest rates do not appear to be tied to any particular tier at all. That's strictly between you, the bank, and your credit report.