I don't share your perception of the druid spell list as being enormously powerful in terms of blasting potential. Looking at the list, I see a few spells with significant combat utility in the bard's range.
Flame strike is the biggest offender, a spell many bards under this system would probably pick up. But they would not gain it until 10th level, and even then would only be capable of doing so once per day baring extreme circumstances.
Entangle is a good battlefield control spell. But while its area is huge, so are its limitations, which in many cases makes color spray or even grease relatively similar in utility. Entangle is limited to areas with vegetation, is not dismissible, and hampers, but does not remove a creature's spellcasting potential. Color spray cannot be used against constructs and undead, and certainly has a smaller area, but is more detrimental at lower levels. Entangle is more powerful in the situations in which it can be used, but it is certainly not leaps and bounds ahead.
Flaming sphere is another good blasting spell for low levels, but hardly an overpowered one. Call lightning and call lightning storm are very similar spells, but they have the limitations of being useful only outdoors. Produce flame to a much lesser extent can be used in a similar method. All of these spells are useful as blasting spells, but much less so than sorcerer/wizard spells. They certainly are not going to turn bards into blasting machines like sorcerers and wizards. They do not turn druids into such.
And of course there is fire seeds. A spell bards would not receive until 16th level. A potent blasting spell indeed, but again hardly a dealbreaker given its limited targetting potential. I've found druids often prefer the area of flame strike anyway.
Ice storm is mildly useful, but as a 4th level spell, it isn't any more powerful than shout, and in general shout is more detrimental to enemy spellcasters.
So let us look at the means a bard has to deal direct damage. The list is limited to summon monster I through VI, shatter (in limited circumstances), sound burst, shout, shadow conjuration, shadow evocation, and greater shout. A bard's ability to deal damage is quite limited until he gains 4th level spells at 10th level, but shadow conjuration and shadow evocation can very easily increase his damage potential in the right circumstances, far more than any druid spell of similar level. Compare that to the druid list up to 6th level. The druid gets produce flame, summon nature's ally I through VI, chill metal/heat metal, flaming sphere, call lightning, flame strike, ice storm, call lightning storm, wall of fire, and fire seeds. In all the bard would gain a few more blasting options at low levels, and nothing significant at high levels besides flame strike, although it could be argued that a bard who wants to blast would have more reliable options with druid spells than bard spells like shadow evocation.
Overall, is it really that big of a difference? Bards are still limited to a very small number of spells per day. That means they certainly won't be outshining anyone when it comes to blasting spells. They can't even outlast a wizard much less a sorcerer or druid. The bard loses a number of enchantments but I have rarely known bards to specialize in those anyway since many effective (like hypnotize and suggestion) can be more easily simulated with bardic music.
I don't think giving them druid spells would require such strong nerfing of all a bard's other abilities (if any nerfing was required at all). The main benefit is that bards have more utility in the average campaign since druid spells, although focused on nature, are less specialized in their effects than bard spells. Is this really a bad thing for a class that is often decried (although falsely IMHO) for being useless in campaigns with little roleplaying? So the bard can deal a little more damage than before in a limited capacity. Does that make the class unbalanced? I hardly think so. The bard gains the ability to deal a little more damage at the expense of some of the more useful arcane battlefield control spells. That seems quite a fair trade and it accomplishes some of the druidic flavor that was present in the 1e bard without requiring a prestige class or other such silliness.
And actually, I wouldn't object strongly to giving the bard a poor base attack bonus. Bards do not have much incentive to spend time attacking anyway.
