So the arguments seem to be:
1) "SIEGE is designed so you never actually get any better at your abilities"
This is an odd idea. Don't all games scale their challenges (and rewards)? As mentioned above, the mundane world stays the same, so if you want to return to your childhood village and rob the mean old blacksmith and you are now a 10th level rogue, you should have little difficulty.
This argument seems rather akin to saying: when I started playing piano I found "chopsticks" to be very difficult, and now after playing for 3 years, I find Beethoven really difficult, so I did not get any better!
A good way to think about SIEGE is to compare it to what it is not. It's not 1st ed. with the DM making an on the spot guess as to the probabilities, and it's not 3rd ed. where the minutiae of builds produces a certain answer to every probability (after 25 minutes of looking through 14 splat books)
Instead, SIEGE says (very roughly): your chances are either excellent, fifty-fifty(50%), dicey (20%), or slim. Then it generally ignores the first and last one. Class, race and relative level will move you around a bit on that chart, but those are the rough categories.
So no finely honed gradations of builds, and no "DM-may-I?" of 1st ed.. Instead you get a rough and ready 50% chance, or 20% chance. Your job (especially if you are a spellcaster) is to make sure you roll mostly the former and your opponents roll mostly the latter
The beauty of SIEGE is its simplicity. Players can play entire sessions without opening a single book!
2) "If you're trying something where you can't add your class level to your roll, you might as well not even bother"
This is somewhat true (but intentional) and somewhat false. It is true that those outside of the class often fail when they try cross class challenges but this is the point of the archetype system.
Almost all rolls you make are general skills, your own class skills, or saving throws, to which level is always added. In fact C&C is a bit too generous in this regard, in that a 9th level ranger is much better at swimming, horseback riding, and juggling, etc. than he was at 1st level even though he may never have done some of these activities during any of his nine levels.
Oddly enough SIEGE has a bit of a problem in the other direction. Because of the power of primes at low levels, low level rogues (for example) making a non prime class skill roll often find their special class abilities overshadowed by amateurs with an "18" stat and a prime in the required stat. The most annoying example being the supposedly ascetic novice cleric who gets a +9 to search checks (+6 prime, +3 stat, +0 level) while the trained rogue is getting perhaps +1 (+0 non prime, +0 mediocre stat, +1 level). It can lead to a shortened life span for your average smug cleric, I can tell you!

This is why some attempts at using other classes skills are not allowed.
3) "It's too difficult to succeed at medium TNs"
As I have noted elsewhere, the challenges are misnamed in the book, giving a mistakenly easy name to relatively difficult challenges. As I wrote elsewhere on these boards:
Aramis wrote:As noted by others, for most challenges you use the HD of the opponent. As a corollary, the adventure itself will be set at a certain "level", tied into the level of the PCs. Thus, challenges of a standard difficulty (e.g what kind of trap on the door?) should be around the level of the PCs. A more difficult one would be higher, and so on. In other words, locked chests (etc.) are more difficult in a 10th level dungeon than a dungeon aimed at 1st level characters. This can be rationalised by the better quality goods hidden within, etc.
However, a problem arises with more generic challenges. Is a slippery tree a 10th level challenge in a 10th level dungeon, but only a 2nd level challenge in a lesser dungeon? Why would a chasm jump have a +10 difficulty just because it is in a 10th level dungeon?
For these more generic challenges, I prefer to use a scale like:
CL 1-5 Standard
CL 6-10 Expert
CL 11-15 Olympian
CL 16-20 Superhuman
These classification titles allow me to think about the kind of person required to beat the challenge. An Olympian challenge could only be completed by that rare super athletic person (or brilliant person, for non athletic challenges). A superhuman one could not be completed by any normal man, and should only be attempted by your Conans and Gandalfs
This also somewhat answers the question about how some things scale (better locks on more valuable treasures), and some things don't scale (a slippery tree is still a slippery tree, whether you are 1st or 15th level, so it should get much easier)
As far as intelligent weapons goes, in my 5 years of playing C&C, reaching "teen" levels on occasion, no player has ever found an intelligent weapon! So the game might be a bit different than what you are looking for. Or perhaps my DMs have rightly feared that any intelligent weapon would soon prove itself more intelligent than most of the PCs in the party, and we would be reduced to a henchman role to various Stormbringers and the like
