In theory, money sinks are to prevent inflation from occurring. Inflation is the devaluation of currency. For example, in Zimbawbe a roll of Toilet Paper costs $417. Imagine if a Super Healing Potion cost 40g! (please note that the situation in Zimbabwe is very serious and not equitable to a virtual game in any way).
Why do mounts constitute a sink?
1) everyone wants one
2) the gold spent on mounts leaves the player economy (more later)
1 and 2 can be insidious, like AH fees or prominent like mount fees. In fact the other likely sink in WoW is the AH fee.
Other items are popular, but must remain inexpensive in order to maintain reasonable game play. Common items could be made into money sinks, but if a Super Healing Potion cost 40g, MANY players would have to spend huge sums of money just to successfully quest or step outside of a city, which would not be fun.
Soulbound items superficially appear to be a sink. Soulbound items create large pseudo-sinks of money, because the items are sold from player-to-player for high prices and then must be vendored for much lower prices. However, the high purchase price is transferred to another player, so the gold never leaves the economy. In fact, this is likely a source of inflation in the WoW enconomy. If high-value items were sold more commonly from vendors, then this would represent a sink (most high-quality items in WoW that are from vendors are relatively inexpensive- time (rep, badges....) is the currency of these vendors).
Is it possible that mounts cause inflation by decreasing the amount of time it takes to gather highly profitable materials such as herbs and ores? Well, herbs and mining represent the only two gathering professions that don't involve significant amounts of time (fishing and skinning being the other two which do require significant time). Enchanting could be included in this list, but requires gathering from the AH and is not dependent upon a mount. So, it is hard to imagine significant amount of inflation occurring from rapid gathering. Swift mounts are 5x as fast as walking.
The big question is: Do Druids need Swift Flight Form?
More than any other class, druids need their Swift Flight Form. As per the discussion regarding druid profession choices, every druid should have herbalism or at least mining. The addition of Swift Flight should allows droods to acquire gathered items at a bit less than 4x the speed of a swift ground mount. The instant cast ability allows for a reduced mount time and for quick getaways from mobs. It is hard to imagine that in Wrath of the Litch King that the increases in mount speed will exceed the gap currently seen in the Burning Crusade (hard to imagine mount speed 9x faster than a ground mount). I suppose anything is possible though. Regardless of the time between now and the expansion, every druid should be working on getting Swift Flight, if they arent already.