There are many good business models which are not impacting much the actual gameplay.
I would classify those in 4 categories, from the less disrupting, to the most:
1) External only: The payment should not affect in any way the gaming experience. This includes donations, merchandizing (tshirts, mugs, ...), web site recognitions (badges on forums, and such)
2) Paid client services: name changes, move a char to another account, email change and such, could be all paid services for few $. In addition access to beta features on test server, participate to special development events, ...
3) In game visual enhancements: engravings on rings, cloaks, signs of guilds, sparkling effects on weapons, guild entrances, beautifying elements already existing in game. This should have no gameplay impact, just visuals
4) In game item skins and models: new models for weapons (example spiked weapon), new textures for clothes (like new robe color), new enhanced models (like a special torso plate with new geometry), new mounts. Those are accessible only to the ones who pay.
5) In game accelerators: experience boosters, buying items, travel quick to the other side of the map,
6) In game powerful items: the sword +200 strength none has, the mount +200 speed none has, the spell to deal 2000 AoE damage none has.
Of the ones above I think I'm pretty much fine with 1, 2 and 3. I start to have issues with 4 as the content will not be available to everyone, and I always wanted the content to be fully available without payments. I have a major problem with 5 and 6, I think those completely disrupt the game, and those will never be applied in a game I develop.
What you think?