Except it's now a run-on sentance, and is not concise language.
You're wrong.
1. It's not a run on sentence, and I'd bet that you don't know what a run on sentence is. A hint here: you can build arbitrary long sentences without making them run-ons. In this case, it's a construction that you were (I hope) TAUGHT IN SCHOOL.
2. Adding ", and" does not suddenly turn a concise sentence into a non-consice one. What it does (in this case) is make things correct, whereas what was printed is incorrect.
'You get X until Y. In addition to this, you get Z while you are W.' It's simple, plain language, concise, and communicates it well.
And is two unrelated sentences, and has departed to the point of no longer being related to the Longtooth Shifter.
This isn't Magic the Gathering, people. It doesn't -require- exact concise language, like you need for a competitive card game.
This is a matter of opinion; mine happens disagree with you and think that on a rules-level, D&D4 -should- be as rigourous as MtG is (and that, in fact, one of the good things 4e did was making D&D more like a CCG).
Lets go back to the original:
Longtooth Shifting said:
Effect: Until the end of the encounter, you gain a +2 bonus to damage rolls. In addition, while you are bloodied, you gain regeneration 2.
This is pretty clearly not correct, because of what I've called out in the second phrase -- it lacks an endpoint, which is sloppy. It's not very important, since this is an encounter ability, and sanity argues for giving the second part an implied "until the end of the encounter". And even if one went for a RAW interpretation, this would mean that you'd get an extra Regen 2 in all encounters after your first one, and only need to spend a minor action if you wanted the +2 damage (which you always will). But it's very sloppy, and unnecessarily so.
Compare to:
Mneme's Longtooth Shifting Reword said:
Effect: Until the end of the encounter, you gain a +2 bonus to damage rolls, and in addition, while you are bloodied, you gain regeneration 2.
That's the simplest surgery one could made to have this work correctly--the duration clause in this case clearly applies to both parts of the compound sentence. Of course, the "in additon" bit is completely redundant; a good way to word this would be:
Mneme's Longtooth Shorter Shifting Reword said:
Effect: Until the end of the encounter, you gain a +2 bonus to damage rolls, and while you are bloodied, you gain regeneration 2.
But the "in additon" does add a certain artistic space, so I'm not that attached to this.