わけではない

The word わけ means conclusion and added with ではない means; "one can't conclude that" or in a more literal sense, "it doesn't mean that."

日本語の試験を落ちたけど、日本語が分からないわけではない。
Although I failed my Japanese exam, you can't conclude that that I don't understand Japanese.

Sometimes instead of using わけではない it is possible to say と言うわけではない。This is often a softer way of expressing the above and avoids directness and a personal opinion.

マイクさんはマンガを読んでばかりいるが、お宅と言うわけではないと思う。
Mike does nothing but read manga but I don't think you can't say he is an otaku.

かねない

かねない expresses when something or someone is capable of something, usually in a negative or extreme sense. The form is stem+かねない

ジョンさんは誰か殺しかねなかったので、警察に逮捕をされた。
Because John was capable of murdering someone, he was arrested by the police.

常にテレビを見たら、病気になって、頭痛がしかねない。
If you watch TV everyday, you are capable of becoming ill and getting headaches.

さえ

さえ is often added to the end of a noun (and sometimes verbs) to express the notion of 'even'. It is used to expresses a more than average or beyond the extent of something. However unlike まで it doesn't go to an extreme length.

日本語は三年間勉強しているのに、「こんにちは」や「ありがとう」という言葉（で）さえ、覚えられない。
Even though I have been studying Japanese for three years, I unable to remember even words like 'hello' and 'thank you'.

あれ？リチャードさんさえ、彼女をできた？！
What? Even Richard has a girlfriend?!

ものだ

ものだ expresses the 'thing that' and supplies information based on the notion that the thing which is being discussed is an inevitable eventuality.

男の人はサッカーを見たり、パブでビールを飲んだりするのが好きだことだ。
Guys like watching football and drinking beer.

For some reason, I always want to just use んだから at the end of such phrases to give it a little bit more of a punch when explaining something that is understood by the listener. A good way to translate it would be, "it is the fact that" or "its a thing that is commonly understood as". Hard to translate the exact nuance into English.

日本語は難しいものだね？