My main problem was class dipping. One level of this plus a dash of that plus prestige class A. All three classes on their own could be equal in power level, but 1+1+1=5. Multiclassing in 3E seemed to make sense and I liked it for a long time. Maybe my players pushed too far into system mastery and power gaming to make it fun for me anymore. Or I should say some of my players. I don't really care if the whole table is running at "5", that's easy to provide a good challenge. It's when the system masters produced "5's" and the other players produced "1's" that challenging the group became more of a chore and at times too overwhelming a task. Since we all had been playing 3E from Day 1 to its "final day" (actually a couple weeks past the release of 4E just to finish our current campaign) there really wasn't much hope that the non-system masters would ever catch up.
Not "odd" because there have been many discussions regarding this very issue.
Most of the imbalance arguments I usually see go the other way.
Usually along the lines of:
1 Spellcasting is more powerful than non magical classes.
2 Spellcasters who multiclass seriously dilute their power and can become seriously underpowered for their level. Dipping for a level or two and remain fairly competitive but the loss of the top spell level seriously hurts power.
3 multiclassing can really screw up your power level in multiple ways by gaining more weak low level powers while missing out on high level powers, losing out on BAB, having powers that don't synergize, conflicting class benefits and restrictions (arcane casting and fighter armor), and wierd save progressions.
Could you give me an example of class dipping that was imbalancingly powerful?
In 3e for example I played a rgr1/mnk1/wiz5 who got tremendous benefits from dipping into both ranger (hp and sword and ranger wands 2 good saves) and mnk (unarmed, evasion, three good saves). He was much more resilient and capable close up than a wiz 7 . . . but a wizard 7 can throw around 4th level spells, has more spell slots, and has 2 more levels for caster level effects in spells (fireball damage, SR, durations).
I was working every power angle I could and it was a good combo for the two PC game I was in, but I knew it was not as strong as a straight caster would have been.
Magic specialization trumps multiclass dipping in 3e power IME.