Steam’s Xmas Sale ’12 – Inevitable illogical investments

[Flint]

Shadow Broker is the best DLC in terms of gameplay content. It and Arrival are the most plot-important ones; Arrival isn't really particularly interesting in terms of its levels etc, but it fills a gap in the story between 2 and 3 (ME3 more or less assumes you've played Arrival) so if you care about that, it's worth a pick. The NPC ones aren't much worth bothering with, the game's already got plenty to choose from and the DLC NPCs don't really interact with you/other team mates beyond their own missions; on the plus side, they also add a tiny bit of ME3 content (glorified cameos, but still). Overlord I've not played. There's also the Normandy wreck DLC which is rather lovely in its own way, but it's a VERY small piece of content.

Arrival offers you the chance to stop an imminent Reaper invasion, after which Admiral Hackett of the 5th Alliance fleet will tell you that you'll have to defend yourself on Earth because of the methods used to stop that invasion. You can just ignore that throughout the entire game, though, and keep going along with your new Cerberus buddies; nothing comes of it. Then in ME3, at the start of the game you're suddenly on Earth, supposedly having handed over the Normandy to the Alliance, the ME2 crew has all left, you've been incarcerated for a few months and... well, I suppose it's one big 'buy our books/comics' situation.

It's more of a timing issue, rather than a book/comic thing. Arrival was the last piece of ME2 story DLC and thus it was 'meant' to be played as the last thing you do before you advance to ME3 (canonically it takes place after the end of ME2's main plot), in which case its ending flows comfortably into ME3's beginning. But because you can buy and play the DLC at any time in the game you want, you can create weird time discrepancies like the one you mention.