A planet defended by 50 or 500 or 5000 units; being indistinguishable between them
Actually, I don't know about other players, but I can quite easily make a difference between a planet defended by 50 units and one defended by 500. I can't say for 5000 though, the highest I've seen in a game was around 1500.
Having said that, I can perfectly understand that it can be frustrating at high unit numbers (ie: above 1k) where it's no longer possible to see a real difference. But to be honest, if it's replaced by a simplistic 'light' or 'heavy', or 50-100, 100-500 and >1000, it won't make much of a difference to me... not much more than the current system, which suits me.