Viva Pinata is surprisingly intense for a gardening game, and one of its few flaws may be that it is too fast paced...
Upon reflection, the only real beef I have with the game is that the work it takes to defend your garden from the sour pinatas really takes away from the enjoyment of the gardening itself - at least until you "beat" the game by taming all the sours and keeping away the ruffians.
This problem is due to the fact that the leveling system is much too fast. Perhaps the super-quick leveling is designed to hold kids attention - and it certainly does that - but problems incur:
1. Because simply planting a tree and watering/fertilizing it can potentially level you up 1-3 levels at a time, as does getting a new pinata to reside (or even just look at your garden), you quickly fly from level to level. This isn't necessarily bad in itself, but...
2. At certain levels, sour pinatas etc show up and basically wreck havoc (making pinatas sick from sour candy, buldozing water areas, smashing pinatas etc). Because you level so fast, it is quite easy to have 4-5 ruffians and sours hanging around your garden all the time causing problems....
3. This is a flaw in the game because: it makes you spend your time defending the garden to a very high degree. (And taming some of those sours isn't a walk in the park - pardon the pun). While defending/taming the troublemakers IS fun, it is frustrating to have a ton of these guys ruining your work. it would be more fun to have these guys appearing over longer intervals of time.
Therefore, the game would be far more enjoyable (and deserve more paythroughs) if the leveling was slower. This way, you could have ample time to work to tame the sour pinatas, create your garden, and enjoy yourself at a more leisurely rate instead of being bombarded all the time just because you planted a tree.