Mike's assessment is spot on and I have very little to add on in that regard. There are, however, a few positive elements that I wanted to highlight. First, the new Raid and patrol area are impressive. It's clear that by distancing themselves from the architecture and limitations of Playstation 3 and Xbox 360, Bungie has started to take advantage of the new possibilities the current-gen platforms offer. I wish that the Light Level grind wasn't as exhausting as it is because I feel like Bungie should be trying to get as many people into the Raid as possible.
Second, the monthly Iron Banner event has been revamped and is a more welcoming experience to all players. The four bounties that are offered up during the event are more straightforward (e.g., get kills with your weapons, get kills with your super) and not only do they provide a decent amount of reputation gain, they also reward you with legendary Iron Banner gear. There's no longer a buff that increases how much reputation gain based on how long the event has been running, it's a straightforward 250 rep gain for a win, and you still get the medallion for a loss, which gets redeemed upon your next victory for 150 rep gain. I was able to get to rank 5 within the span of about 4 hours (spread out over a couple days). For someone who's not a big fan of Destiny's PvP, it was a really welcome change.
The last thing that I like with Rise of Iron, though it is a mixed bag, is that all of the "Year 2" gear you acquired from the Taken King is still viable. Instead of just locking it at last year's max Light Level (335), you're free to infuse your King's Fall Scout Rifle all the way up to 385. The bummer here is that if rumors of Destiny 2 are true and all of your current character progress is left behind, it's all pretty ephemeral. There are legendaries and exotics that I have fought hard to earn and while it may seem weird to have any kind of attachment to that stuff, they often represent memories of time spent conquering the raids with friends, or completing a particularly challenging quest. I can only hope that Bungie won't wholesale go back on their original sales pitch of "carry your character across this 10-year thing."
Bottom line? If you're a fan of Destiny and want more content to dig into, Rise of Iron is a good enough filler. It's not as good as The Taken King, but it's also better than the Dark Below and House of Wolves content from Year 1.