It wasn't really a surprise that cross-platform play wasn't allowed on Destiny. While it is plausible that was forced upon by technical limitations, the fact is that it was a design decision intended to level the playing field.
"Regardless of where the reality is, there's definitely a perception among gamers that better hardware means you have an advantage," explained Bungie engineer Roger Wolfson. "We don't want to have to enter that fray, so to create the best, most level playing field, both actually and perceptually, we separated it by platform."
To prove his point, Wolfson noted that a sniper aiming at far away enemy while playing on older hardware might see his target as 2 pixels while a player using a modern console would have his target in twice the detail.