If the quests are unchallenging, then the problem is one of world design, not quest design. Breaking into a home and stealing something for someone else is no less fun than breaking into a home and stealing something just for you. The only difference is that when you're doing it as a quest, you get paid for your services. Some players enjoy doing these quests, just as some players enjoy repeatedly fighting bandits, or collecting oddities, or decorating their homes. If the quests are boring, then don't do them, or at least don't overdo them. I doubt radiant quests can adequately replace individual, hand-crafted quests, but they aren't intended to either. They appear to use rather simple templates, so I do not believe they cost Skyrim much in the way of individual quests, not enough to complain about. A fair portion of players wanted generic, filler quests to supplement the finite, exhaustible stock of other quests, and that is exactly what radiant quests provide.
But that still doesn't change the fact that the only thing that comes even close to the kind of gameplay I'd expect from the Thieves Guild (sneaking and stealing) are these repetitive dull radiant quests. The regular quest hardly has any sneaking and theivery to speak of. The way things are going so far, (I'm up to the part where I have to go to Markarth to get a translation for some ancient book) I don't feel like a thief. There are no consequences to not sneaking around (ex.failing or forfeiting a bonus) and the way things are going I think that you could play this questline with a warrior type and still be fine.
The aforementioned radiant quests are only fairing a little better in the whole "feeling like a thief" aspect and that isn't saying much. Oh great you want me to break into so and so's house and plant evidence that bears no meaning to them and in the end accomplishes or for that matter does nothing (they don't get arrested or anything)? Cool!
Bearing this example in mind I have to ask: Why is this a better alternative to having individual quests that have multiple solutions (ex. routes you can take or methods that can be used) and that get harder and more complex (ex. you can't be seen by person X, you can't kill guards or else you'll fail the quest or forfeit the bonus, you know a consequence for not acting like a thief) as you proceed in the questline?
Also I semi-roleplayed a thief (before doing any of the radiant quests and not having any knowledge of what they were like or even were) and it actually a lot more fun than the radiant quests. Here is why: The reason why I was doing so was to get rich and have lots of valuable potions. It also made me feel like a thief. With the radiant quests however what is my incentive? The guild is going to get better! Oh really? Are you suggesting that doing these repetitive quests (planting evidence, stealing item Y, forging notes in tavern/inn X) over and over again (until it all culminates into the theft of an Elder Scroll) are going to make the guild better? What? Besides not doing these quests bears little effect on the guild itself. Its not like if you don't do them and decide to concentrate on the main questline the guild itself will fall apart (ex. losing members) or anything. So if nothing happens whether I do them or not and said quests are repetitive (to me at least) then what is the entire point of doing them over and over again?
*Honestly the Elder Scroll theft sounds interesting but I'm simply not willing to sit through a bunch of boring radiant quests (which really just involve me fast traveling to the designated location and doing something that, at most, takes me 2 minutes and then fast traveling back to the guild) just to reach it. I'd rather have individual and unique quests that built up to the theft (preparing gear, reconaissance, info gathering etc.) then these radiant quests.