Most criticisms of Sign that you'll hear are valid; It's slow, the characters can be pretty insufferable from time to time, and its success has caused spiritual successors or follow-ups that one might like better. The score is really quite good, though I could see where over 26 episodes, one might get tired of hearing the same music cues. Further, in spite of the brilliant watercolor backgrounds and hand-drawn nature of the show, I'd not defend Sign as being visually dynamic or particularly exciting artistically.
Where Sign stands out is in drawing you in to its world (helpfully enough called 'The World') and its conflict. The music work and backgrounds are a start, being in and of themselves works of art that can stand alone. The characters interactions and bonds are vital and develop organically in ways that make sense given the individuals' psyches and goals. Leads towards fixing what's wrong in The World normally take an episode or longer to mature, so when the party does strap on their adventuring boots for a plot-relevant dungeon romp, the payoff seems greater.
There's a lot of good character work here, with Tsukasa being so divisive and annoying because he is the up-his-own-ass woe-is-me Shinji Ikari that perhaps we all had a phase of being. Everyone in the show is vulnerable, though those vulnerabilities are what drive them towards their noble (or sometimes ignoble) ends. Dropping hints on each character's trauma takes place at a pace that might be seen as too slow, but at least avoids the trap of seeming like a rushed exposition dump.
There are some flaws here that can't simply be handwaved as a matter of taste, hence why I don't think it's a masterpiece and or that anyone who objects to it is a philistine. There's some filler and fluff to pad out the methodical pace the show wants to go at, characters will swap loyalties readily to the point it can be hard to track what characters are working towards, dialog can sometimes be more accurately described as a whirlpool than a back and forth towards a point, and elements of the plot that are played up as really important may not get a commensurate amount of screentime.
All that said, Sign is a work that speaks to me a lot. It is without doubt a bit of bias on my part that its flaws are ones I more readily accept and its virtues and potential are things I normally thirst for in anime, making for a perfect storm where such an obviously imperfect show can be so dear to me. Give it a watch yourself; even if you're not sucked in by it, its shadow stretches long across the MMO anime genre, making it as much a cultural piece as a show in and of itself.
