I have mixed feelings about this. While like some others I have reservations about any new foray into the world of pay-for items, this doesn't have to be a bad thing. So here are some things to consider:
1. Is experience an advantage? People have mentioned that there is BOA gear, but BOA gear is advantage that only some people have. A brand new player isn't going to have a full compliment of BOA experience boosts, so really this elixir would actually bring more of a balance for people who want to experience end game without spending large quantities of time to get there. And really, isn't the majority of what we do and pay our subscription fees for the stuff that happens at max level? Experience boosters provide no benefit once you get there.
2. Will items like these be an excuse to charge for things that should be automatically included in the game? Blizzard will ultimately have to answer this question, if not with words than with actions. It's a definite fear (or concern) that rather than do things like the recent reduction in XP needed to level for MoP, Blizzard might instead simply have a sale on experience boosters at the store. Nobody likes to feel they are being made to pay additional fees for things they should already have, so this is a real Wind Bomb sitting in the room. Tread carefully.
3. Many services that could be offered in the game could actually create a positive benefit for those of us who play long term. Personally, I think the line would need to be drawn in the area of "niche" items. I will explain.
A "niche" item would be an item that a small percentage of the player base as a whole would use, both before and after the addition to the store. This automatically precludes any item that offers player power or performance. Such items would be used by any serious player, as well as a lot of semi-casual players, so they would become a requirement of anyone not wanting to be labeled as a noob (or maybe a "freebie"). However for example if there was an item such as say, something that gives you the appearance of having vampire fangs. If it were available for free in the game, some people would use it. Others would not. If you added such an item to the store, the results would be the same; a few players would buy it and a great many would not. This is the perfect balance you want for real-money purchases in-game.
Basically, the best kind of items for real-money purchases would be the exact same types of items available via the loot cards from the CCG's: Special mounts (which are already available), companion pets (also already available), and fun knick-knacks like the Foam Sword Rack that enhance the player experience while having absolutely no impact on the actual game experience itself.
Edited for typos.
Edited by Lubricious on 7/3/2013 6:29 PM PDT