1. It is my long and legendary privilege on this forums to use colours for quoting..... expect a letter from my laywer soon, suing your for copyright violation.... 
Consuming memory - You can turn Steam off you know - Indeed, but not if you want to play a game requiring it.
Consuming CPU time - See above - In which case, it consumes both CPU and memory that could be better used for your game.
Needing time to launch - Your computer needs time to boot does it not? Internet pages need to load yes? - Booting of the operating system is necessary for primary system operation, loading of web pages is necessary to receive new data. Steam on the other hand needs over 10 seconds to transfer a few lousy bytes of login information. And more importantly, the software serves no purpose or at least often it does not serve the currently requested function. I want to play Sins.... not watch a steam window going up. By the time Steam finishes loading, I could be in the main menu of the game already. Now, if you want to play online, it is of no concern, but for singleplayer the steam client does not serve any purpose in the background. Yes... 10 seconds are not much..... but for nothing.... ?
Updates of the Steam Client itself can become annoying - Well I'll be damned if they want to make their service better - They should start with making client updates go silently in the background. 
Surprise game patches that break savegames if you dont read the forums regularly - You do realize you can prevent automatic updating of games - If it wasnt so bugged... yes. 
Sometimes a less than stellar user interface - It'd be worse if they never updated - True, but who had the brilliant idea of implementing a web browser that does not support an adress bar? Makes finding a page you have in the client very difficult to find in Firefox. 
I think that I forgot the most annoying thing about Steam though:
Steam Fanboys! 
