Insanely high dps, great AoE damage with shortbow/caltrops, everything about shadow refuge, best mobility profession behind only ele, thieves guild makes any 1v1 against class moot, numerous escape methods etc. Of course I'm not saying they can do all these things at once but the fact that they have access to all this while other classes only have access to one or two of things at all. If the others had as much capability as the thief then this wouldn't be an issue but they don't so it is. The only drawback to thieves are their paper defenses but those don't really matter when you can ya know escape effortlessly.
Other classes should ideally buffed to the point where they have just as many potential options. Also, your post made it sound like they had all these available at once. I'm glad you're not that retarded.
Variety and viability isn't bad. It isn't OP. It's fun. Taking that from one class because others are lacking is retarded design.