"Has DLC gone too far?"
Yes.
Oh, you wanted me to qualify?
I liked the old-school system of releasing a (full and clearly finished) game, wait a year and release a big expansion (repeat for a maximum of two expansions total) then work on sequel. This means new campaign/levels (at least 67% estimated play-time of original), new units/weapons, preferably a new mechanic of some kind, and a release price no greater than 50% of base game's release price.
Alas, those days are gone. So, off the top of my head:
[Manifesto begins]
1) No releasing incomplete games for full price then grabbing more money via DLC. Not cool. Either make a full game or a free-to-play game, you greedy bastard. This includes day-one DLC. Also see the Asura's Wrath incident.
2) No in-game plugging of DLC (Dragon Age, I'm looking at you). Exceptions might be made for ads on the main menu maybe, depending on how it's done. "Tasteful" is your watch-word here. Strictly no real-money shops ingame (special exemption for free-to-play - but again, keep it tasteful).
3) I don't particularly like preorder bonuses, mostly because in a post-Aliens: Colonial Marines industry I'm generally against the practice of preordering. I could maybe tolerate preorder DLC if it's non-exclusive; ie give weapon skin x for free to those who preorder from retailer a, and weapon skin y to those who preorder from retailer b, on the conditions that all such items can be purchased by everyone else after release, and that the other rules of the manifesto are generally adhered to. Timed exclusives - fine, if you really must.
4) A season pass is all DLC. All of it. Looking at you, Borderlands.
5) Release a reasonable amount of DLC then move on to the sequel. That means you, Sims.
6) Cutting features of previous games/expansions in the sequel, so you can release them as an expansion again - generally not cool but depends on circumstance I guess. Just don't take the piss. For example, since pets were introduced in a Sims 2 expansion, I see no reason why a seemingly-simple feature shouldn't have been in Sims 3 vanilla game, at least in fundamentals (especially since Sims 3 had the item shop to expand beyond basics).
[Manifesto ends]
As for whether I was ever put off a game by DLC - the last (but not only) example was when I was about 5 seconds from pressing the button to buy Payday 2 but paused to read one more article about it first and discovered their plans to release (3?) DLC packs in future. F*ck that, I'll wait for a GotY or something. Maybe.
Also, this is what goes through my head every time I have any in-depth thoughts about "DLC":
Every. Damn. Time.