One of the questions might be how often people get to allocate funds to different budget items. The example Bill mentioned is defense spending. Suppose I feel 30% of the budget should be spent on defense. If my vote allocates 30% to defense and then I find out a quarter of the voters allocated 0% to defense I might want to change my allocations.
For people that vote, but actually pay no taxes because of low to no income the prospect of voting 100% to social services maybe terribly attractive. Only a budget reallocation by others can compensate for these "votes". Again, re-cycling the budget allocation might be needed to correct perceived "imbalances". On the other hand, frequent votes on the budget makes it difficult to do any long term planning by the agencies that do the spending.
There are many "single issue" folks and aggregators to match. For example health care for seniors, free abortion clinics, public education, training and supplying well armed militias. On average these may work out, but you can't be sure.
For those that want to allocate directly to budget items keeping up with events will be crucial.
The idea is still half-baked.