Story
I wasn't expecting much out of this one, mainly due to the short run time. But I have to say, in the end I actually appreciated the brevity. With the exception of a few parts, I think it was about as long as it needed to be. There was no padding or filler—just a compelling plot that moved right along. My biggest issue with BLs is that they usually aren't about anything, aside from relationships, which I often don't find enough to really propel the plot. Therefore, I get quite bored. Not at all with this one. Here there was enough setup, with the mob backgrounds, to actually give the story some direction. It did focus on the relationship, but not at the sake of the story. BL may not even be the correct category for this. It was definitively about two men, not boys.

Personally, I don't like cute fluff, another common trapping of BL. Long Time No See was not cute, in the least. It has a sweet, tender moment or two, but it was actually pretty gritty, and a bit dark. That being said, I was so relieved at the ending. It could have been a tragedy, with the mob plot being what it was, but thankfully they decided to give this one a happy ending. I guess my only story complaint was that they could have fleshed out the mob and the rivalry going on there a bit more. It was very clear, but it could have had a touch more depth.

Acting
The acting is probably what stood out the most, overall, for me. The cast was very small. In as far as named characters go, I wasn't complaining. Another sticking point for me with BLs is when they jam-pack the cast and end up not giving enough screen time to anyone. That certainly wasn't the case here—no pointless secondary or tertiary couples. The mob, again, could have done with a few more members or goons, or whatnot. It came across a bit empty, and might have been more threatening had they showed more of the operation or manpower. The two mob bosses (if that's what they technically were) that they did show, were well acted. 

The fight & reveal scene was really well done. It could have been over the top with weeping and shouting, but both of the actor's opting for subtlety instead was what made the scene hit really hard.

Intimacy/Heat level
This might not be on everyone's review list, but when intimacy isn't done well, it takes me right out of the story. The couple had really great chemistry, and I was really surprised by the level of physical intimacy they displayed. No fake kisses, and seemingly no discomfort with any of the scenes. In the world of BL, that's really refreshing. The only other Korean BLs I've seen have been very poor in the intimacy department—scant scenes, fake/close mouthed kisses. This couple managed to be sexy and sweet at the same time, while looking real. And this is probably the first BL I've seen with partial nudity. It wasn't done in any kind of gratuitous manner. It complimented the scene in which it appeared. 

Their intimate scenes also weren't overly staged. Sometimes it's clear that the actors have perfected the timing and rhythm of their love scenes, and that's not necessarily a bad thing, but it can come across as very rehearsed. Here, especially the first intimate scene, they just seemed natural. 

Characterization
I separate this one from acting, as it has more to do with how the characters were written. I found both leads to be engaging characters with fairly believable motivations, although, this is one of the areas where it would have been nice to have just a bit more. I would have liked to know a little more about them. I liked the twist of Gi Tae knowing Chi Soo is his target, but not knowing (at first) that he's also a writer of on-line serials that he really likes. As far as a reason not to kill Chi Soo right away, I thought that was actually really clever and worked. From there it's clear they fall for each other, and that's why Gi Tae stays his hand further. Small detail, but I liked it.

Another thing I really appreciated is that the characters were "on equal footing," so to speak. There was no dominant partner, and they stayed away from the whole husband/wife, top/bottom stuff that I loath. Physically, they were a really good match for one another as well. Yeon Seung Ho may have a baby face, but he is actually really tall, but he was never seen as cute or childish, even before we knew who or what he really was. They made Gi Tae's character just as strong and resilient as Chi Soo's (also making him a hitman certainly helped in this category). Whenever the two male leads are on equal footing, I really, really appreciate that.