This is something I talked about with few of my friends but but they did not saw any problem with this, while I see it as the game's biggest flaw. In Spellhold the game presents you with a choice of whether to take a ship or portal. This is in my opinion really badly designed choice from the game point perspective. I have 2 main reasons for this:

1) It's not a choice. There is no drawback to choosing ship, and there is no advantage in taking the portal. None. You gain more gameplay time, more loot, more experience and more plot. When you take portal you are bypassing all of this and loosing content.
2) The game lies. You are told that one way of transportation (ship) will take you safely to mainland while the other might be faster but harder and might EVEN take you to underdark. Player given that information will obviously choose portal. It's even amplified by the fact that your previous travel by ship is just a paragraph of text, while your previous travels by portals took you to interesting and fun places.

When I first played the game that's what I did. I chose portal. Duuh! I'm not stupid! Easy way? No thanks, I'll take the hard way please and thank you. I'm a real gamer damn it! Then when I found out from a friend of mine the truth I felt cheated out of content.
I think this is bad because there is an hidden optimal choice and it basically requires luck or metagaming. (It's like that best sword in Morrowind that required you having an item, an exact 5 digit amount of gold in pocket, being vampire AND talking to a speciffic person.) I think that the player should either not be able to choose or the portal path should have exclusive maps and quests of its own. What do you guys think?

Actually, from a speedrun standpoint, the correct choice is the other one. By choosing the portal, you reach the end of the game in less time then if you chose the ship. (Notably, gaining gameplay time is actually a negative in a speedrun.)

(Of course, this is assuming you are doing a "glitchless" speedrun where you disallow polymorph teleportation. If you allow polymorph teleportation, you can skip almost all of Spellhold (including plot critical scenes) by using the portal when you first get there.)

In other words, you are only considering one type of play style; that which seeks to see as much of the game as possible and end up powerful by the end. For different playstyles (such as speedrunning), the optimal choice.

This is true - I did not consider different play styles. But then again, once you set your priorities you are still left with no choice. The only correct answer is just reversed. Hah, this is still better than Tazok who can be killed I think 4 times in the saga ;)

Then again, if you look closely, there are some inconsistencies with Imoen in Baldur's Gate 2.

1. If you play the tutorial, Imoen's thief skills (Set Traps in particular) are much higher than in the actual game. (There is a good gameplay reason for this; the developers don't want you to fail while still learning the controls, much like the reason Xan has 25 Intelligence in the tutorial.)

2. CI Imoen (the one you recruit in Irenicus's Dungeon) has a mysterious item called Imoen's Belt. That is actually an incredibly powerful item that protects her from most forms of death (including 0hp death). The game provides no in-game explanation about the belt; it's just there and there is no mention of it disappearing later in the game. Also, if you teach her any spells, Spellhold Imoen (the one you recruit in Spellhold) will have forgotten the spell by the time you rescue her.

3. If you do manage to kill CI Imoen (for example, by stacking Contagion 3 times and having her read a Scroll of Weakness), Imoen will still show up alive in the cutscene after leaving Irenicus's Dungeon, and the dead Imoen will still be out of your party.

4. The cutscene that plays when you leave Chateau Irenicus can be skipped, though it is apparently not easy to do so. If this is done, you can keep CI Imoen, go to Spellhold, and then (assuming you don't skip Spellhold) rescue Spellhold Imoen and have 2 Imoens in your party.

5. The tutorial party can be imported into Throne of Bhaal, at which point I am pretty sure you can get a duplicate Imoen (and Jaheria, maybe Minsc (don't remember for sure)) in your party. (This does let you play with Branwen and 25 Int Xan, but keep in mind that the tutorial party all has BG1 style proficiencies, and I am not exactly sure how that works.)

Welcome to warp zone. (I feel like you'd fit in great on on the elder scrolls forum, trying to talk n00bs out of fast travel.)

You miss half of BG2 if you take the main story seriously and play accordingly, the portal isn't the only occasion.

(This post contains major plot SPOILERS out of necessity.)

Of note, there are actually a few long term advantages to the third, unintended choice (using polymorph teleports to enter the portal early).

Advantages:
1. You get to keep Yoshimo for the rest of the game. I believe you can even keep him in Throne of Bhaal if you complete Shadows of Amn. (Choosing Import Game will cause you to lose Yoshimo because you aren't supposed to have him there, and it's possible to import a pre-Spellhold save.)
2. You get to keep your Bhaalspawn powers for the rest of the game.
3. You don't have to give up a stat point in the dream. (Is it possible to avoid this by Shapechanging (via Limited Wish) into a mind flayer and eating the demon's brain?)
4. You get to skip part of the plot, which means fewer cutscenes to sit through (good if you're doing a speedrun).

Drawbacks:
1. You don't get to recruit Spellhold Imoen.
2. You don't get any of the treasure in Spellhold. In particular, you don't get the Bag of Holding that you find there.
3. You don't get the ability to turn into the Slayer.
4. If you are interested in the plot, you skip major plot points if you take this route.
5. Because you did something the developers did not intend, there *might* be strange bugs. (Anyone know for certain?)
6. Actually taking this route is not that easy; you need to do 2 polymorph teleports in a row (I don't know if the Iron Giant form, which requires Shapechange, is necessary, but its large size is useful here), or you need to use the Ctrl-J cheat. (Fortunately, you only need one character to reach the portal.)

Until you ask him to wait in the Copper Coronet and he promptly gets destroyed.