Being one of the "logs on every day to try to reach the cap as fast as possible, spending day and night grinding up to the cap" kinds of people, I don't tend to think that games -do- get topheavy all the time. Admittedly, World of Warcraft did in some respects, especially when newer players are still trying to learn and get screamed out of groups by impatient, irritable high-level types (a sad reality, but one that exists). The solution is to keep the ceiling low, I think, and make creating alts something to do and enjoy past a "get this up to the high level so you can do more stuff at cap".
On the other hand, World of Warcraft does offer a -LOT- to do at the cap, between dungeons, raids, achievements, professions, and even gold farming for some, which makes the game nearly endless.
Personally, I'd say a bigger worry would be more about how to keep the mid-game attractive and interesting. It's sad to say, but you can only see the same cutscene, hear the same dialogue, or read the same text so many times before you want to just glance over it and get to something more interesting and new (e.g. at-cap content that's just been released).
Creating a "donut effect" in the level system (lots of low level characters, lots of high level characters, and almost no mid-level characters) can really drag a game down into tedium if the developers aren't careful. Admittedly, a good portion of this rests on the player's ability to actually play through the mid-levels, but to keep it interesting is something the devs should make sure stays consistent. Given that the world of Rift is going to be dynamically changing, however, it seems like that won't be -too- much of a problem, though.
Dranei Holy Priest, lvl 12 -- Suikun