Mostly I'm thinking about three things:

1. Availability—Not only the issue of the limited number of metropolitan areas you can get it in, but also signal quality within those areas. 3g is spotty within supposedly "covered" areas (just like regular cellular access, except worse ), there's no way WiMAX will surpass 3g in this regard right off the bat.
2. Cost—I haven't researched this point as much as I should, but $40/mo (the price I've heard quoted) is a bit steep for single-device access (well, if you factor in a true cellular plan as well, though VOIP is probably an option).
3. Battery Life—3g certainly can't manage an all-day always-on connection, so it seems highly unlikely that WiMAX will be able to manage it (although I imagine powersaving could be better managed with it than with a negotiated Bluetooth connection).

In the end, I think a lot of people are overhyping what it is and what it offers. Not that it isn't a great piece of tech that's a great first-step in the always-connected revolution, but it is a first-step.