Unfortunatly I can not really reccomend the following places but its a starting place that you can ask more about.
I have never eaten Ethiopian food, although I would like to try it some time. But I have heard of Habash, it is located on 8 HaNegev street.
There is also another one called "Ethiopia" on 13 Alenby. Ask around about those two places.
I have rarely eaten couscous out of the house because I get the real deal at home from my wife. But I have heard Interesting reviews about Casserole like MarkC Said.
There is also Dr. Shakshuka, a Libyan/Tunisian/Moroccan eatery in Yafo. I have never eaten there but hear about it a lot. Ask around.
I have no idea which is the best couscous is in Tel Aviv, but here are three good options:
1) Ha Mitbachon, Rabbi Akiva 18 - near the Allenby entrance to the Carmel Market - this little middle-eastern diner is a local institution
2) Casserole, 3 Lillienblum St., near the corner of Rishonim St. in Neve Tzedek. In a beautifully gentrified old neighborhood, nice for walking.
3) Ba Li Kooskoos, 144 Ibn Gvirol, above Jabotinski. Unfriendly location as far as parking. They also have a branch on King George St. which is easier.
I'm not familiar with Ethiopian restaurants. A million years ago someone took me to a place called on Anjera, I think on Allenby St. I find the cooking extremely limited - more of an ethnic than a culinary experience. If you're REALLY adventurous you can take a walk on Newe Sha'anan St. near the central bus station where all the Sudanese workers and refugees live. It is grungy, but lively, and perfectly safe, at least during the day. You will see a lot of hole-in-the-wall eateries without any sign, and a few pots bubbling on the stove. I make no representations as to the sanitary conditions of these places.