In many ways I like T&T more than D&D. It's much, much loser than any version after Holmes for one thing. It has been my convention game for a year now. Ken St. Andre is a huge Lin Carter fan (as am I).
Why don't I adopt it as my primary game? Three reasons, in order of importance: increasing stats, tone, and combat. T&T uses wholesale stat increases to convey experience, raising stats either by the new level number, half of it, or twice it (depending on what stat you choose). That can quickly get silly. The tone of the game itself is silly, more silly than I'd prefer. Finally, and least important, is its system which is much more group than individual combat.
However, it does have two things I love. First, there is no xp (adventure points) for treasure as it is its own reward. Second, there is its saving roll (SR) system. They also combine to give an alternative to playstyles from OD&D.
Instead of saving throws, ability checks, or skills (those show up in 5.5 but use the same system) there is a unified saving roll system. The GM gives you a level of saving roll to make. As explained you roll 2d6 and subtract it from the SR value (20 for 1st level, 25 for 2nd, and so on). If your luck is greater than the result you pass otherwise you fail. Of course, this is the same as luck + 2d6. But what if you have a luck of 7? Well, you still have a chance because doubles explode. So in theory a loser with a luck of 3 has a chance to make a 7th level throw with a target of 50. You can also see why those increasing stats are important.
However, there is more than just a luck roll here. The rulebook mentions using other stats and it is a convention in T&T play that a character may get a SR for any attempted action. Finally, SR garner experience points, succeed or fail. In 5th edition the amount of xp is the roll time the level while in earlier editions it was the amount rolled for success and twice the amount missed by for failure. Here is where the two systems converge. OD&D rewards thought and trickery by giving xp for treasure obtained regardless of combat. It encourages relatively safe action to get experience. T&T encourages daring doing by rewarding attempting risky actions.
I'm thinking of adding SR to my S&W variant rules allowing the characters to add their level as well as the stat and rolling on 2d10. Lower levels will be a tad easier but without the stat increases higher levels may be harder. I might try 2d6 as well and see where we go. Of course, I will remove treasure xp in favor of SR xp if I do use SR. I think it will encourage more of the game I want to see.