One is something I alluded to on my personal blog the other day, which was about my experience playing a warlock in World of Warcraft. I like the class, but when I play that character too long, I get tired of pressing the same buttons in the same order every single fight. The only things I vary are (a) which curse to use, which only changes if I'm fighting a spellcaster, (b) whether to default to Shadow Bolt or my wand after I do my three DoTs (which depends on my mana), and (c) whether to put some Drain Manas in there (depending on whether the mob has mana and how my stores are doing). Frankly, it gets boring.
There have been iterations of Fourth Edition where we've had the same problem. Fundamentally, it's a problem you encounter whenever your resources are perfectly renewable. Some characters in Tome of Battle have that problem, although with combat in D&D being more dynamic than PvE combat in WoW, there are always things that encourage you to mix up your pattern. But you tend to default to using your best power, then your next-best, and so on down the line.
For that matter, NPC spellcasters in 3e have much the same problem, and PC spellcasters can fall into it as well. For an NPC who doesn't care about resource management, it's simply the best strategy to lead off with the best spells and work on down the levels. Heck, that's why our new stat block format lists higher-level spells first.
When you have the right balance between powers that refresh all the time and powers that are more limited, the game becomes more interesting. Strong power design also helps. When some of your powers are per-day, you're constantly asking yourself, "Is this the fight where I break out this big gun?" When your powers are well-designed, you also ask the question, "Is this the right round to use this power?"
Look at the 3e barbarian. At low-level, rage is a once-per-day ability. The key question for the 3e barbarian is which fight is the one to rage in. (Unfortunately, that usually translates to, "Is this the fight where I get to have fun?") As he gets higher level, it becomes more like a per-encounter resource, and he uses it every encounter. It lasts long enough to cover the whole encounters, so it's actually no longer an interesting choice. It's more like a default state. If it were better designed, the barbarian would be asking himself, "Is this the right round to start raging?"