Carlos Kleiber's Tristan und Isolde is an intricately nuanced affair, smaller in its overall sound, but more human in its conception than many other recorded performances (and certainly than most stage performances). "Smaller" is actually the wrong word: it never lacks punch, and it rises to the same overwhelming, lyrical crescendos that you'd expect from late-period Wagner. But it doesn't look for climaxes where there aren't any, and it dares to be intimate.
Kleiber's intention to realize Tristan in the finest of detail is clear from his choice of singers: neither
Margaret Price (Isolde) or
Brigitte Fassbaender (Brangäne) ever sang the work on-stage, but each had a voice of breathtaking beauty (if not typical Wagnerian size) and expressive imagination to match;
René Kollo (Tristan) was showing signs of wear in his voice, but had the perfect feel for Wagner; and
Kurt Moll was a far more lyrical King Mark than is often heard. For those reasons, it may or may not appeal to the hardcore Wagnerite who puts stentorian vocalism first on his/her wish list. But it's hard to argue against the recording on musical grounds.
Kleiber's pacing -- fast enough to avoid bogging down, but never too fast -- and his attention to expressive detail will call your attention to elements of Wagner's orchestration and tightly knit motivic constructions that you haven't heard before. The preludes to the acts are, themselves, worth the price of admission:
Kleiber lets them unfold naturally, caressingly, so that when they finally build to great heights of sound there is a feeling of having arrived there honestly. The absence of forced showmanship actually makes climactic moments stronger -- intense, slow burns instead of sudden flashes of fire. There are occasional balance problems between singers and orchestra, but otherwise the sound is nearly ideal.