1) There is no plot.
Uh, okay... Let's recap the Half-Life 2 storyline and then discuss what we learn in Episode 1.
Half-Life 2: Following the events at BLack Mesa, the Combine invade and take over the Earth. An indeterminate time later, Gordon is reintroduced. He meets his old friends from Black Mesa after being detected by the Combine as an unauthorized prescence. Following his escape from the city, the safe area is raided by the combine and Dr. Vance is captured. After an adventure along the coast, he meets up with Alyx at a prison facility to rescue Vance. The rescue is botched and damage to a teleport causes them to teleport back to City 17 in a weeks time. During the course of that week the resistance uses the raid on Nova Prospekt as a rallying cry to stage an uprising. Since Vance is located in the citadel, Gordon makes a beeline for it. After fighting numerous street battles he arrives at the citadel and fights his way to the top. After briefly being captured by Breen, he follows him to the Combine teleport reactor. With the help of Alyx he blows up the teleport reactor, critically damaging the citadel. G-Man pulls Gordon out of time and space before being blown up in the resulting explosion.
Episode 1:
The Vortigaunts use there own teleportation abilities to pull Alyx out of the citadel, and Gordon out of the G-Man's grasp. They are both deposited in the wreckage at the foot of the citadel where they are rescued by Dog. After setting up communications with Vance and Kleiner, they discover that the Citadel is going to go critical far too soon for them to escape the blast radius. As a result they reenter the citadel in an attempt to temporarily restabilize the reactor core so as to allow enough time to evacuate the city. While in the damaged citadel they encounter what might be the slug creature that Breen is seen talking to at the end of Half-Life 2; whatever it was it was loaded into some sort of pod. Deeper in the citadel Alyx and Gordon discover that the destruction of the teleport reactor caused a chain reaction among the world's other citadels that has prevented the combine from teleporting in reinforcements or even communicating with combine forces outside of Earth. Apparently, the explosion of the reactor core will cause a rip in space-time that will allow the Combine to send out a message, possibly a distress signal, hence why the Combine have been trying to accelerate the explosion. After Gordon stabilizes the core, Alyx is able to get a copy of the Combine message, however translation of the message will have to wait until they can hook up with Vance and Kleiner. Additionally, the receive communication from Mossman, who is searching for some sort of project at what appears to be an arctic base. The transmission ends with the appearance of some sort of previously unseen Combine creature. The pair escape the citadel. After fighting through some zombie invested tunnels, they reach the City 17 surface. A prerecorded message by Kleiner on the City 17 TV screens confirms that the Combine on Earth have been cut off and now is the time for humanity to regroup in preparation for the Combine's inevitable return and counterattack. After a series of adventures, Alyx and Gordon reach Barney who is coordinating the evacuation of City 17 refugees through the trainstation. After evacuating the refugees and having a close call with a Strider, Alyx and Gordon escape via another train. As City 17 fades into the distance the reactor goes critical, tearing a whole in space time above the citadel. Additionally, the pods that might contain the slug creatures are seen being fired from the citadel and spreading out over the countryside. There is a final explosion and the screen fades to white.Personally, I'd say there is quite a bit of plot to Episode 1, certainly more than Half-Life 2. There are clear objectives, an impetus to do so, elements of the world are fleshed out, and numerous hooks and questions are left for the next episode.
3) The beginning is terribly dull/tedious and the continuation from HL2 is poor (no explanation)
I thought the explanation of the transition between HL2 and E1 was pretty clear. Maybe you didn't like the explanation but the intro definitely explained what happened between the end of HL2 and the start of E1.
The opening levels arn't action packed but I thought they looked incredible, had great atmosphere and had some interesting puzzles to solve.