As the friend who has spent many hours lamenting on what we perceive as the 'flaws' of DnD, I feel I should put my two pence worth into this discusion.
In my oppinion D&D 3.5's greatest strength is that it is so closely based upon AD&D.
As a child I loved basic D&D and AD&D, and I have to admit I like D&D 3.5 a lot.
The game system was and still is relatively simple. This can be a good thing as a new player can pick it up and get playing with relatively little investment of time and effort on their part.
Game play is relatively speedy as there is no need to endlessly cross reference tables to discover what happens, players and GM's alike can be reasonably confident that action A will result in action B so on and so forth.
The game can be a lot of fun to play.
Almost everyone I know (or know of) who play roleplaying games has at some point or another played D&D (or one of it's varients).
However I personally feel that the first and biggest place the D&D system falls down is the flat level system coupled with it's linea probability distribution curve.
Most of the flaws in the D&D game can be lived with, gotten around, looked fondly upon as a quirk of the system ... etc.
However the current level system and the power differential between 1st - 10th - 20th level characters is so great that it makes the necessary suspension of disbelief trying.
As written D&D 3.5 is designed that after 13-14 encounters at the appropriate challenge rating, a Party of Adventurers are supposed to aquire their extra +1's and Dice of Hit Points.
3.5 is also designed so that characters can freely chop and choose what class to advance in by simply adding to gether all of the +1's, so a Wizard is not limited to just being a man in a dress who likes to wiggle his fingers and cause the dress' of Tavern wenches to unbutton.
Unfortunately it is this very design principle that makes the power disparity inherent in the original AD&D system even worse in 3.5. At least in AD&D PC's stopped getting HD + Con Bonus after 9th level, could never get and AC better than -10 and would never hit AC0 with less than a 3 (before modifiers) hell Monsters would never hit AC0 with less than a 6.
I guess what I am saying is good framework needs limiting factors, PC's should always be able to advance but as a PC becomes beter at something it becomes harder to get better.
To my mind D&D 3.5 is a game with out limits.