The introduction of a seventh stat (beyond Str, Int, etc.) in Terminal Space got me thinking of the general idea of stat expansion. There's been a crapload of games that expanded the general range of stats to achieve finer distinctions, such as splitting Dex into Reaction Time and Agility or whatever. What I like about Terminal Space's Tech Level stat is that it introduces something new, a new focus for characters and therefore the overall campaign. Another great example of this is the Wizardry stat introduced in Tunnels & Trolls in the little tin box edition. The Wiz stat solves the T&T conceptual problem where all wizards were built like Arnold Schwarzenegger because previously Strength was your spell point stat. More interesting to me is that by having Intelligence split from natural magic ability (a la Chaosium's house system and Encounter Critical) you can have a spell-dude that has a lot of raw power but not a lot of smarts.
It seems to me that Comeliness, introduced in a Dragon article and later canonized in the original Unearthed Arcana, was pretty much a failure of a new stat. The fact that it was basically split off from Charisma indicates to me that it was never needed to begin with. Did anyone ever have a situation where they thought it was important to mechanically clarify that someone was an ugly charmer or a dull beauty? I just couldn't get worked up about that.
Speaking of Unearthed Arcana, its pages also contained a percentage-based random social level chart, so that you could determine whether your PC was an upper-class twit or a no-good bum. Had that chart taken the form of a Social stat rolled on 3d6, I think maybe people would have taken it more seriously. The Honor mechanics in Oriental Adventures and HackMaster would feel more palatable to me if they worked on a 3d6 scale. Even going up a down more often than the other six, contextualizing Honor as one seventh of the core PC abilities would better fix Honor as a Big Honking Deal. As it stands, Honor all too easily melds into the background of secondary and tertiary stats on the charsheet.
So while the basic six stats look sufficient for most D&D-type purposes, Terminal Space makes it clear to me that introduce new basic character abilities can work. The key seems to be sticking to a 3d6 scale like the others, not overlapping with existing abilities and, most importantly, only introduce a new stat if it says something interesting about your campaign.
One final aside: I don't like Luck stats very much. The normal dice rolls are where Luck enters the PC's life. Having an additional Luck stat seems like metaphysical double dipping.