Tom Ewing fills everyone who wants to know the answer to that question in, at Pitchfork*:

Thrills were what 2000AD traded in. It even calls individual stories "thrills" and its stated goal is the delivery to its readers of "thrill-power." Every British comic started in the 1970s and 80s had its own jargon, designed to make the kids buying it feel part of a club. 2000AD was no exception: It had this notion of thrill-power, and it had a supposedly alien editor, Tharg the Mighty, a guy in a green rubber mask with a telephone dial stuck to it. Tharg really came into his own later in the comic's history, but the credo of thrill-power was there from the beginning. The voice inherited by Millar is in full song in the first few years of 2000AD-- crude, overdriven, hilarious, and fantastically exciting.

Oh, and there's this decription of a cover of the mag. Which I want right damn now:

One legendary 1978 cover catches the tone perfectly-- illustrating the story "FLESH II", about time-travelling dinosaur farmers. The cover is dominated by one enormous speech bubble: "NO! Please let me drown BEFORE the GIANT SCORPIONS get to me!" It's crudely drawn, but the picture-- people in water; scorpion (giant)-- isn't the point. The idea isn't to pique the reader's curiosity, or suggest a tricky situation which a hero will have to escape from. It certainly isn't to spark empathy (hmm...if I was in that situation, would I rather the drowning... or the scorpions?) No, the cover is pure rampaging thrill-power, designed for grins and sensation and nothing else.

*Well, filled in, almost two years ago. I found it in the comments section for Graeme McMillan's Brave and the Bold Review at the Savage Critic. I even nicked the second quote from that thread.