The user interface feels completely different. It may be the 3D-ness or maybe the sounds. It felt very odd to select things and to use the hot keys. There was as if there was some sort of response missing. Maybe it's that "boing" sound that you hear for every action you make in BW that isn't there. I would probably get used to the unit response sounds after a while. Another thing is that I accidentally added units to building groups and then when I tried to use the hot keys to control the buildings I would instead be controlling the units. So my scouting SCV would end up going back home because I wanted to change my rally point. It might just be that the ctrl key + # has some more functionality. Or maybe it was just a strange side effect of having all commands be in the QWEASDZCX region of the keyboard so I wanted everything to be hot keyed at 12345 as opposed to using 67890 as well.
The game feels very fast and very slow at the same time. Slow because the map is huge. It feels like you're miles away from your opponent. Research, building and training times seem very long, but the buildings themselves die very fast when under attack. Unit and building animations are fast and twitchy. I won't say anything about response time. I'm guessing it's close to normal BNet latency but it's very hard to tell with the units moving like they do.
There was something incredibly weird about your income as terran. The MULE really makes a huge impact where you feel like you get too much minerals and not enough gas. The rememedy for this seems to be to take gas earlier than what seems right unless you were going marine heavy early. It was really easy to get into the groove and to guess what you could afford after a few games so you kept your minerals down. As such I feel the game is a success. It allows you to play freestyle without feeling like your play is too suboptimal.
Some units are really tiny, some units are gigantic. This is visually pleasing I guess, but it causes some minor problems when trying to select a small unit hiding beind a thor. The units themselves behave very oddly. They are like magnetized balls moving in a clump if you don't put any effort into them. So I didn't feel like I was in full control the whole time.
Units feel like they really counter each other completely. I'm not sure if that is just a balancing issue or if it's designed to be that way. Certain units will feel useless against certain others (I mean it's as if they weren't even there). Some units seem to be fairly good at everything. Some units feel bad at everything. It makes me remember my early Starcraft days when I thought vultures were crap. Maybe the stalker will eventually find its place in the world. I felt like I was mostly being rewarded for having the right units, not so much what I did with them. So that tells me that scouting and build order will be paramount and the micro part of the game is less critical. Of course, build orders are all about cutting corners where you can so perhaps the micro part is more important when you get to the higher ranks.
The game is very dominated by the main armies. If you devote too much resources to anything besides your main army (expanding, teching, harassment forces) you are in a position where you die if your opponent attacks you. So a lot of the time you are in this awkward place where you just sit in your base massing up and spying on your opponent to make sure he does the same. You feel like you want everything in one lump so that if his lump comes at you your lump will survive the battle. Once his lump is gone you automatically crush him if you reinforce properly.
Terran has changed a lot. Air power has greatly increased. Banshees are so good that I feel bad for my opponent when flying around with them. It also feels like to succeed you need to integrate all the different unit groups instead of either choosing all out mech or all out bio. Mech and bio are more similar to each other than what they were. All in all, I don't mind the change as long as the race stays distinctly different from the others. I can't really comment on if it is because I only played terran.
Altogether it was a very fun experience. I got to try out a lot of cool ideas. Surprisingly a lot of them worked out well. Probably it was as much because of chance as because they were actually good ideas. Me and my friend ended up winning about twice as many games as we lost at gold league level. I feel like the auto-matchmaking system did a good job giving us a suitable challenge. I can't say much about balance or how good of a competitive game it is, but it definitely has the right feel to it. I just hope as many different strats and tactics work at the higher levels as they did when I played.