Mounts

Mounts are one part of the game where I feel that spending real money gives you a considerable advantage over a free player, especially while levelling. The basic mounts you can buy for gold are very slow and offer only minimal protection against being dismounted. You might be able to acquire a slightly better one during an event, but the really good ones generally only come from the Zen store. At max level you can grind out Astral Diamonds to buy a Zen store mount from another player, but while levelling you're basically hosed if you don't want to shell out real cash.

Admittedly not everyone will perceive the increased dismount protection the same way, but I found it very noticeable. On a basic mount you get knocked off if you take three hits within five seconds. On a purple quality mount five hits are required. This may not seem like much of a difference, but you have to consider that most groups of enemies in the game consist of three to four mobs. On a basic mount, your chance of getting dismounted and being forced into combat at a bad time (which might possibly even lead to your death) is therefore pretty high. On a purple mount, you'll be able to continue riding even if every mob in any given group gets a hit in.

I know that for me, buying a fast mount made a huge difference and made the mere act of moving around the map a lot less stressful. If you're going to spend real money on anything at all, I recommend a mount.