One-off account upgrades(opens in new tab)
The one-off upgrade offers a limited feature-set to new players that expands when you pay a one-off sum. In Team Fortress 2, buying an item, any item, at any cost, will upgrade a free account to a “premium” one. Free players have a backpack limit of 50 slots, doesn't have access to rare and cosmetic items, and have access to limited selection of crafting blueprints. Buying anything from the Mann-Co store expands the backpack to 300 items and removes trading and crafting limits. Star Wars: The Old Republic's free-to-play transition added more severe limits, constraining free players to handful of space missions and dungeons per week, forbidding new players from sprinting until level 10. Those limits could be lifted with any purchase of more than $5 on the in-game store.
The cost of transitioning to a less limited set-up is often minor, the intention being to familiarise players with the game's shop and, in some cases, get players to enter card details. Team Fortress 2 is entirely playable with its free-to-play limits in place, but The Old Republic's draconian restrictions leverage player frustration to incite a purchase. Not good. If you're looking to familiarise players with a store, then Guild Wars 2's tactic of gifting XP boosts and items provides a much better experience.
There is something to be said for one-off payments that unlock everything. Players put off by the complications juggling ongoing micropayments can instead just buy the game in an ordinary way. In Card Hunter, you can play a flat $20 fee and unlock all of the missions. This lets players treat the free-to-play element as a demo, and still gives players that don't want to spend a big lump sum a way to play for less money.
In short: Contrived limits like the The Old Republic's give new players a handicapped experience, which makes it unlikely they'll stick around, especially when the competition includes MMOs like Rift and Lord of the Rings Online. These offer a huge amount of playable content without charging for basic features.
Account Buffs(opens in new tab)
Buffs give a temporary percentage increase in the amount of gold, XP, or other desirables that the player can earn through regular play. It's another example of microtransactions allowing players to pay to reduce the time spent between rewards. Unlike energy, though, buffs are a bonus applied to someone who pays, not a penalty against someone who doesn't. That's a key difference in their philosophy that, for the most part, stops them being exploitative.
For them to work, it requires a careful balancing of item prices and levelling progress. There's a strange psychology here. If a game is enjoyable, then a lengthy spell between rewards shouldn't be a problem. But if progression and upgrades are built into the DNA of a game, having to wait too long for them can feel frustrating. In games like World of Tanks, progression is swift to begin with, but slows greatly as you advance. This deliberately plays on impatience to incite a purchase, and is a classic example of game design serving a monetisation system rather than the player. If a game is perceived as a grind, then a buff becomes a requirement rather than a bonus.
It's not just currency that can be boosted. In the case of Card Hunter, your account subscription provides you with an extra piece of loot for every quest you complete. It's an upgrade that neatly sidesteps the balance problem. It doesn't feel like a significant loss compared to the 2-4 rewards you get in regular play, but a guaranteed rarity makes for a nice bonus for those who do subscribe.
In short: In free-to-play games, XP boost items can be symptomatic of an overly sluggish levelling curve, but for patient players there may never need to go near account buffs. If a game is entertaining enough, putting a lot of time into it shouldn't feel like a chore.
Mini-DLC(opens in new tab)
Blurring the line between microtransaction and full-fat DLC are these purchasable packs of extras and bonuses. Rather than a free-to-play focus, you'll generally find these attached to AAA releases. We're talking the added profile portraits of Crusader Kings II, the silenced sniper of Deus Ex: Human Revolution, the Air Propulsion Gun of Just Cause 2, and the squirting blood of Shogun 2 (to name just a fraction of a percent).
As a practice, these mini-DLC packs are the most variable in quality of all the microtransaction methods. There's nothing inherently wrong with providing fans with a fun extra to flesh out a world they're enjoying, but too often they're created with little attention to balance or value. The worst, inevitably, were once pre-order bonuses leveraged as an incentive to tempt early buyers. As well as the aforementioned sniper rifle, DX:HR's Tactical Enhancement Pack added 10,000 credits at the start of the game, effectively destroying many of the game's early purchasing choices.
For non-narrative led, systems-driven games, mini-DLC seems to fare better. Crusader Kings 2's profiles and music packs focus on aesthetic improvements in a game about strategic depth, while Civ 5's extra civilisations expand user specialisation, without changing the core of the game. But that doesn't mean that other types of games can't utilise mini-DLC in a way that adds something enjoyable for the user, that doesn't make the original game feel lacking without.
In short: Mini DLC like the Total War blood pack and the inventive Just Cause items are a bit like professionally built mods. There's a perception that mini-DLC is stuff that's been held back from the final game to screw a few extra bucks out of players, but more often they're ideas on the developer's big brainstorming board that they can't justify putting resources into during the development of the main game. Mini-DLC is easily ignored, at least, but beware of pre-order DLC that gives you guns and gadgets at the very beginning of a game like Deus Ex - they could ruin the balance of those opening hours.