The 2e way of handling damage and spells was, IMHO, bull




. If someone won initiative and hit you, even whith a handy sewing needle, the spell you wanted to cast would be wasted. A battle between spellcasters was nothing more than hoping you win initiative.
As there is no static start of the round anymore (initiative is rolled once, and then the order in which the combatants act usually doesn't change anymore), the old "recent damage" rule was taken out. Instead, as soon as you start to cast, you provoke an attack of opportunity. If you take damage from that or from anything else while you are casting, you have to make a concentration check or lose the spell. It's not automatically lost anymore. You can cast defensively and voluntarily make a concentration check in order to avoid the attack of opportunity, but if that fails, you still lose your spell.
The Attack of Opportunity's not the only way to stop you from casting: If the spell has a casting time of one full round, it will last until just before your next turn, and you can be attacked normally. Also, others could ready actions to attack you as soon as you started to cast a spell, and that would work even with ranged attacks or if you cast defensively! And of course, there is the possibility to counterspell your magic (although that's only really effective with the right feats). So its not too hard to disrupt a spell, but it's not to easy, either!