They're noisy, require LOTS of wall power; generate LOTS of heat(especially OTL ones!) and LOTS of distortion(even considering even order ones) making the playback far from real. Very often the soundstage will seem like sounds from far away. Also as tubes about to become bad, these unpleasant effects multimultiply including some unwanted echo, RF or EM interference or microphony. Tube components indeed sound different, but they're just toys and nothing serious no matter of reputation or credibility of one or another manufacturer.I missed these comments, which are mostly untrue.
Distortion: our amps are fully differential and balanced from input to output and so don't make much of the Even ordered harmonics as they are canceled throughout the amplifier (not just at the output). At full power the THD is typically aroun 0.5% (mostly 3rd harmonic), IMD is around ).005% - 0.01%. As tube amps go that's pretty low distortion; for an amplifier with zero negative feedback that's **really** low distortion. The lower ordered harmonics (including the 3rd) are those to which the ear seems to pay little attention; its the higher orders where the ear is much more sensitive, more so than even the best test equipment. In this regard tubes in general outshine solid state for low distortion! The ear converts all forms of distortion into tonality. The word audiophiles use for higher ordered harmonics is 'bright'. They also use the word 'harsh' and similar.
Heat: this is a product of class of operation, not the number of tubes. If our amps are left on standby all day at the end of the day you can put your hand on the power tubes and not get burned- they just aren't that hot. But turn them full on from stone cold and within a minute they are way too hot to touch. Solid state amps that run class A for a similar amount of power make a similar amount of heat.
Soundstage; This is something tubes do very well; the perception being that this is something that tubes do much better than solid state. One of the reasons for this is greater low level detail. In the soundstage this manifests as room ambiance, which often makes for a greater sense of depth. Some less experienced listeners might perceive this as 'further away'.
If tubes really were just toys they would not still be in production they way they are, half a century on after being declared 'obsolete'. You don't have to know anything about the technology to understand why: the market likes them and wants them and so they are still around. The reason the market likes them is simple- its easier to get a tube amp to sound good to the human ear than it is to get a solid state amp to sound good to the human ear. This is because tubes are more linear and so its easier to get them to not make the sort of distortions that the ear finds objectionable.
In case its not obvious, its how amplifiers make distortion when they are not overloaded that is the name of the game (and FWIW, tubes are much more pleasant than solid state when overloaded as well...)! If you understand how the ear/brain system perceives sound, then it is possible to take advantage of the ear/brain perceptual rules to make a better sounding amplifier. Most of what we know about how the ear/brain system processes sound was discovered in the last 35 years or so, so if you are working with design rules from the 1970s or earlier your amp is going to not sound as good.
Its worth pointing out that most test and measurement concepts stem from the 1960s and 1970s, which means that to build a good amplifier often means that it might not measure well against those rather outdated notions. The old testing regime is not based on how the ear hears but instead is all about looking good on paper.