Revenue Enhancing Devices/Analysis

Why Should I Buy This Stuff?
Revenue Enhancing Devices can appeal to players in lots of ways. Here's the main ones.

Cosmetic Enhancements
Things that don't alter the gameplay or add to it are nonetheless be very important to people enough for them to part with hard cash to get one of their own. Unique cosmetic items can enhance a player's social status among his peers, appeal to his collecting or customising instincts or just plain looks cool and does funny stuff.

Because cosmetic items don't modify gameplay they tend to be most accepted in gaming communities and they cannot damage the game's balance. However their inability to give players an competitive edge makes them correspondingly less desirable.

Convenience
Adverts that pop up all the time? You can pay to remove 'em. Is your farmland now so big that it's a chore to plough it all? Here's a disposable item that does it all for you, only a dollar for a pack of four. Is that sword a 1 in 100 drop from a unique mega-monster? It's also on sale at the local store – for real money.

Progression-Speeders
Strongly related to Convenience items, these allow the player to bypass much of the grinding. Includes letting the player purchase important resources like money, items that increase the odds of getting item drops or that double XP from killing monsters, premium crops that offer better yields and so on.

Progression speeders are not too controversial among players or damaging to gameplay balancing as all they do is speed up a gamer's normal advancement through the game – unless normal advancement is incredibly slow and unpleasant otherwise. Often progression-speeding methods are justified in that people with more money than time can get to enjoy the game as much as people with more time that money, and as long as the former can come up with enough cash the latter get to play the game for free.

Extra Playtime
In games that use energy or similar methods to limit how much you can do per day you can often buy something that lets you extend that limit. One-shot items like energy drinks and food are usually employed rather than permanent time-boosters as they unbalance the game less (and keep people paying regularly).

Power Superiority
These devices make the player character faster, better and more powerful than a nonpaying grinder could ever hope to be. Examples include unblockable special attacks, combat units that upset the existing balance and have no weaknesses (except to other premium units...) and mounts that give him additional attacks.

Premium-enabled superior abilities are of course rather controversial in multiplayer games, especially balanced, competitive ones.

Unique Content
Access to abilities, areas and other things denied to nonpayers. Includes Expansion Pack and Downloadable Content lands, VIP nightclubs, alternative customisable loadouts and subscription-only game servers. The ability to own or customise a game's content may also be restricted to those who can afford it, and if that game is Entropia Universe you could be talking a lot of money! Somewhat related is content that used to be available to all but has since been retired for the game, except for still being available in shops for real money.

For premium-only content in a game that advertises itself as free, see Allegedly Free Game.

Unique Content can easily blend into Power Superiority if they give a player an advantage that his nonpaying opponents cannot match, eg. he can access premium tunnels that let him walk right into an enemy fortress.

Real-World Stuff
Sometimes online transactions reap real-world benefits – access to gaming tournaments, trading cards and whatnot.

Rewarding Awesome People Who Don't Pay
While not this trope per-se, players may earn some of the above content not by paying the game's developers but by helping them out in some other way. For example, by getting friends to play (very common with Facebook games), installing other games by the same developer or publisher (often nets the player a small amount of premium currency, very common on mobiles) or achieving prominence in the player community.