I think that's a good way too. Let's try to throw in some ideas. Probably you already thought about these ones but let's just say anything we can come up with, you never know what can happen:
Feeling special: Give some sort of special treatment to those who pay. Call them like commanders or something like that, give them a cooler interface, make them feel like they have a deeper, richer experience (even though you are just selling comfort and shiny pixels).
Personalization: We've seen this all around. Maybe they can customize their avatars, or their title; premium users can (I think they definitely should) have an icon besides their name... I don't know. Let them be different than that bunch of guys who don't care about the game as much as you do, you badass general you.
Be part of the game: Maybe there's a cool feature that only they can do, or maybe anyone can do it but only by being premium (or by paying a small amount of money) you can post your results in public. Like, I don't know, if this was some Monster Hunter thing I'd say let them make their own recipes (or, say, design spaceships) and publish them so other players can use them and they can feel good about other players using them, or let them vote about some changes in the plot, some moving NPCs, stock market, anything. The idea is that since they take the game more seriously than the free players, the game trusts them to take a little bit of control over the game experience (theirs and other players'), and let them feel responsible for the other players (so they will want to come back to exercise that responsibility).
Make gifts: I don't know much about this one but I think some studies say that a lot of people actualy pay to be able to make gifts to other players. So there's that.
Time savings: This one is pretty dangerous since time is usually the main resource in this kind of games, but maybe you can make them save time in things that don't give them a competitive advantage. Like some kind of bonus XP for the first 30 levels of WoW. (Okay this one is simmilar to selling comfort).
Time away protection/assistant bot: Maybe they can go out for a weekend and they want some tasks to be made. Maybe they want some actions to be made even today at a certain time. Maybe they just want a SMS to be sent at their mobile phones when something happens. What made me leave oGame a long time ago was that everytime I went out party I forgot about my battleships and somebody would tear them apart. Probably not me, but maybe if you could tell someone to pay a dollar to give them directions and not have to worry for the rest of the weekend they would appreciate it.
Collections: Maybe there's some random goodie that they get with each payment so they can make a collection! Everybody likes to collect stuff, right? They can put them on a shelf somewhere and let people see it or something.
Okay that's all I can think of right now. I understand that all of these ideas need some polish and that they will take more work than just putting a "get more energy" button, but I think it's a start to a much better system.
Sorry to ask a question and then flee the country for a while
Thank you everyone for the great suggestions.
Would Team fortress 2's drop/purchase system count as Valve using monetization (pourposely to get money)?...... The more I think of it that way the more it seems like. I am a major fan of Valve's work on that game, and I would hate if it was since that would ruin their reputation in my mind. I know that these large companies need to make money...
Would Team fortress 2's drop/purchase system count as Valve using monetization (pourposely to get money)?
Mm, I don't think so. Not spending money doesn't really restrict you from meaningful content, and while it does take more time to unlock certain weapons without spending money on crate keys, I can't really see it as a cashgrab function.
I know I'm digging up ancient history here by posting in this, but this video floated back into memory recently, as I've been playing a game that uses this kind of system over the past couple days.
I'm talking about Spiral Knights, BTW.
They give you a 100 energy cap, which takes about a full day to replenish. Energy is used up by dungeon raiding (10 per floor), and in making new gear (which is cheaper than buying new gear outright. The energy cost varies)
I have to say that I absolutely hate energy systems with a passion. Sometimes I want to really invest myself in a game, and I can't do that as easily if every action either takes significant amounts of time to complete or drains an energy bar that refills at a very slow pace. Particularly if the game has a storyline to it, these limits really irritate me.