Bioware didn't think this through.

[Timmibal]

Except when Liara becomes the shadow broker.

...Which happens wether you purchased LOTSB or not. Likewise Arrival ends the same way no matter how you play it. Those who have not purchased it have missed absolutely nothing. The only DLC in ME2 with any choice in the outcome was Overlord.

Spoiler

And I highly doubt the fate of some 'tard savant who can speak geth is going to make a hill of beans difference when you already HAVE the Geth onside against the reapers. I'd put money on that being an e-mail or a Citadel News reveal if they bother at all.

 

[Jonesy911]

the side quests/ characters that would have been there wont be /thread

It's not that, ME-3 beginning will follow after the event of the Arrival DLC.

Or maybe, if you haven't played the DLC, the beginning will be different?

I've played the Arrival and it really doesn't explain anything useful about the gap between 2 and 3

Spoiler: Arrival spoilers

Essentially if you don't know, the DLC ends with Sheperd destroying a mass relay to keep the reapers at bay for a little longer

The events of this DLC can and probably will be easily explained by nothing more than a ten second cut-scene at the beginning of ME-3. If you don't play Arrival ME-3 won't be affected at all. Arrivals only real function is to give the people who love ME-2 and are looking forward to ME-3 a little more action to help bridge the gap.

Except for that whole part where:

Spoiler

Shepard kills a thousand Batarians and is called to court for it.

edit:
I mean, it could just be summarized in a cut scene, true.

Exactly because there was no option given, the game forced you to do it. Cut scenes will easily cover the dlc people missed