Counting the beta, SC2 will be two years old in March - and the game's interface has barely developed over that span. While the lack of LAN support is the major issue for most people (as frequently demonstrated in tournaments with connection issues), I feel that the general feel and usability of the new battle.net UI are just as frustrating for the average user.
 
Sure. We've seen some improvement with chat being added. But it's telling that we had to ask for that in the first place. And we have vague promises for the map marketplace / arcade to fix the completely broken custom game scene with HotS. But there's so much more.  
 
Personally, the game's UI discourages me from playing on a daily basis. When the new battle.net was developed, we were promised that we'd "never want to be offline again". But every single time I log on, I feel utterly alone, maybe play a game or two, and then leave again. Both in SC/BW and WC3, I'd idle around forever, hang out in channels or waste time in custom games.
 
So what`s different?
 
Communicating and socializing is too hard.
 
Chat. It still sucks. Simple commands like /w outside of games would make life tremendously easier. Real channels would be great. Or sending messages to my whole friends list. At the moment, any act of communication is a hassle; I either have to navigate through my friends list (which could be a lot more accessible too, by the way) to message people; I have to clumsily navigate through menus to send messages to other folks (or add them to my list); I even have to click through menus to ignore or report people.  
 
Everything could be so much easier with a decent chat interface, yet we seem to be stuck with the farce of a system we currently have.
 
Clan support (+clan channels). This rocked in WC3. You had a place to meet with friends and clan mates, and you could even moderate your channels. You actually had a sense of community while online.  
 
Automated tournaments. Where are they? Why was such a great feature removed? Despite losing quickly in most of them, I often played them in WC3. You got to meet new people, had a competition that wasn't ladder, and had an incentive to play for the portraits.
 
Simply playing the game can be hard at times.
 
A working custom game system. Despite the chaotic first impression, WC3's system worked. All the SC2 popularity system does is kill any incentive to play maps that aren't on the first two pages. Also, Blizzard's attitude of blocking development instead of encouraging it have led to a custom scene that's pretty much dead. The vocal criticism of the ridiculous limits on map developer freedom have died down because most developers have stopped developing and caring.
 
I had hoped to see countless maps by now, exploring the boundaries of the map editor and creativity, but all we have are some DotA clones, some TD maps and nothing too exciting else.
 
AT vs. RT issues in team games. While team games are looked down upon by many, they nontheless enjoy a large following. The strict separation of AT and RT from WC3 was scrapped for SC2 in favor of short queue times, but despite having a matchmaking system implemented, it's still prone to abuse and frustration. Things like 2/3man ATs signing up for 3/4man RTs is just one example of many. Currently, the team ladders are a sick joke and the results / ratings and the general activity reflect the utter neglect with which this issue has been treated.
 
Analyzing is overly hard as well.
 
Shared replay watching. It was such a great feature in SC/BW. Requested since beta, and we'll probably never see it.
 
Statistics. Where are they? Why can't I see my race-specific %, or my odds on certain maps? And why are lower league players belittled by having their losses hidden, even when everyone knows that those people still here after two years must have realized that it's a competetive game and losing is part of the deal?
 
The whole league system. Sure, we all know how and why the MMR works right now. But why scrap the system that worked so great in WC3? Right now, we can't compare anyone (regardless of whether you think ladder matters or not). Grandmaster league helped, but only if you're looking at pros. I can't even judge whether I'm better than my buddy or not, because we're both in meaningless divisions of pointless leagues, and our MMR is invisible.  
 
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Things went wrong when Blizzard decided to develop the new B.Net from scratch, and to employ a casual game / console developer for the job. I have nothing personal against Greg Canessa, but I feel it shows that he developed for PopCap and Xbox Live before. That by itself needn't have been a bad thing, but why for the love of all that is holy did everything that worked well in previous games have to be removed? Hell, the better part of Blizzard's fame came from releasing games with fantastic interfaces and great functions, yet SC2 is a step back (or several) compared to 2003's WC3 or even 1998's SC.
 
And why all this? Well, of course because it works for Blizzard. Enough people were happy with what they got, not enough people cared. The folks who just bought the game for the campaign obviously didn't bother with the menu much, and the competetive 1vs1 crowd basically just needs a working matchmaking system (which is great, btw, and the only redeeming feature). But there's a huge number of players interested in socializing, custom maps and analyzing their play. Those that haven't left have grown complacent or jaded with Blizzard's ignorance.
 
The adding of chat and the scrapped forum real-ID feature have shown what players can achieve when the voice concerns; the complete lack of development in the meantimes is a great sign of how little will suffice for the majority to accept their fate.  
 
This game could be so much better.